Nadia Begum counted stitches the way other people counted heartbeats. The sewing machine at station fourteen of Apex Garments Limited thrummed against her sternum, a mechanical pulse that had replaced her own rhythm sometime in the autumn of 1984. She was twenty-three years old, had been in Hackney for eleven months, and had not seen the River Lea since the day she arrived, though the factory stood not two hundred yards from its bank. The water that fed the dye vats came from the Lea. The water that filled the drinking taps came from the Lea. The water that ran green through the gutters on wet mornings came, like everything else in this corner of East London, from the Lea.

On the morning of April 17, 1985, Nadia noticed that the dye vats were producing the wrong colour. The indigo was coming out petrol-green. "The water's off," said Mr. Chowdhury, the floor supervisor, and he said it the way a man says rain is coming — as if water turning strange was simply another English inconvenience, like the miner's strike dragging into its second year or Margaret Thatcher's voice crackling from the wireless with news of more pit closures. Nadia bent back to her work. She did not know, could not know, that the green was spreading.

Two miles downstream, where the Lea emptied into the Thames at Bow Creek, Arthur "Artie" Tibbs sat on the edge of the derelict West India Dock and watched the river change colour. Artie was sixty-one years old, a docker who had not docked anything in four years. The containerisation that had killed the London docks had killed his purpose too, and now he spent his mornings watching the water the way old sailors watched the sea — not with hope, but with the grim satisfaction of a man who had predicted destruction and been proven right.

"It's gone green, then," said a voice behind him. Artie did not turn. He knew the voice. Jimmy Chen, the only Chinaman still living in the old Limehouse quarter after the opium dens had been cleared and the council had built the estates, came to sit beside him every Tuesday and Thursday. Jimmy was seventy if he was a day.

"It went green last week," Artie said. "It's greener today."

"The green," Jimmy said, and spat into the water. "My grandfather told me about green water in Canton. 1893. Fifty thousand dead before they understood it weren't cholera."

"This ain't Canton."

"No," said Jimmy. "This is the Lea."

The punk squatter called herself Toxin. Her real name was Emma Coulson, but she had shed that name the way a snake sheds skin when she moved into the abandoned council flat on the Somerford Grove Estate the previous winter. The flat had no electricity. It had no gas. It had, however, a working tap, and Toxin had filled a bucket from it every morning to wash the safety pins out of her hair and the eyeliner off her cheeks before another day of squatting, shoplifting, and attending gigs in the basements of Brick Lane curry houses where the air smelled of cumin and anarchy.

On April 18, Toxin filled her bucket and the water was green.

She stared at it for a long moment. Green water. Green like the hair of the girl who sang for Rubella Grave, the band she'd seen at the Amersham Arms three nights ago, moshing in a basement that had once been a Victorian cholera ward. Green like the algae that grew on the canal path behind the estate, the Limehouse Cut, which everyone knew was toxic but nobody did anything about because it was Hackney and Hackney was where things went to rot.

"Bloody hell," Toxin said, and carried the bucket to the smashed window anyway. She needed to wash her hair. The water was green. Everything in Hackney was green — the mould on the walls, the weed growing through the cracks in the pavement, the sickly light of the streetlamps that the council had stopped replacing after the rate-capping. One more green thing was nothing.

She was wrong, of course. But Toxin was nineteen years old and trust in institutions had been beaten out of her generation the way the police beat the miners at Orgreave. She had no framework for understanding that a river could hold a grudge.

David Prentiss worked for the London Borough of Tower Hamlets Environmental Health Department. He was forty-two years old, balding, with a wife who had left him for a social worker in Islington and a daughter who only called when she needed money. His job consisted of writing reports that nobody read, attending meetings that nobody remembered, and issuing citations that nobody enforced. He was, in the language of the department, a "liaison officer," which meant he was the man who got sent to talk to angry constituents so the people with actual authority did not have to.

On April 19, David received a phone call from a woman who lived in a narrowboat on the Lea. "The water's gone wrong," she said. "It's not just green — it's got a texture. Like velvet. And there's something underneath it. Something moving."

David drove to the river. He parked his council-issue Ford Escort on the towpath and walked to the water's edge. The woman was right. The Lea was not merely green — it was a green so vivid, so lurid, so unnaturally bright that it seemed to glow from within. And beneath the surface, something pulsed. Not a current, not a tide, but a deliberate, rhythmic movement, as if the river itself had developed a heartbeat.

David took a sample. He drove back to the office and filed a report titled "Unidentified Biological Contaminant — River Lea, Hackney Section." He marked it URGENT. He placed it in the out-tray. The out-tray was emptied once a week, on Fridays. Today was Saturday.

The report sat unread for six days.

Eleanor Hartley was a senior water quality engineer for Thames Water. She was thirty-seven, a graduate of Imperial College, and one of only three women in her division. She had spent her career fighting against the kind of institutional negligence that had allowed the Thames to become an open sewer a hundred years earlier and was slowly, painstakingly, being reversed. She had testified before parliamentary committees. She had been shouted down by MPs who accused her of "environmental alarmism." She had watched the Clean Air Act slowly begin to clear the London smog and believed, with the fierce conviction of the recently converted, that the same could happen for the rivers.

On April 25, a water sample from the Lea treatment plant arrived at her laboratory in Reading. The sample had been collected by an automated system, one of hundreds that Thames Water operated across the catchment area. No human had seen it. No human would have seen it, except that Eleanor was working late, as she always did, and the sample was one of sixty-three that needed processing before the weekend.

She put it under the microscope and nearly dropped the slide.

The organism was unlike anything in her textbooks. It was a cyanobacterium — blue-green algae, the bane of eutrophic waters everywhere — but it had been altered, deliberately, along lines that suggested genetic engineering far beyond anything the 1985 biotechnology industry could achieve. Its cell walls were reinforced with something that looked like chitin. Its reproduction rate, extrapolated from the sample, was exponential. And embedded in its DNA — she could see it, somehow, in the way the cells arranged themselves — was a pattern. A message. The algae was not merely growing. It was hunting.

Eleanor called her supervisor. The supervisor was at a dinner party in Henley-on-Thames and told her to "deal with it on Monday."

Eleanor called the Environment Minister's office. The receptionist asked if she had an appointment.

Eleanor called David Prentiss at Tower Hamlets Council, because his name was on the report that had finally, six days late, crossed her desk. The report that had been filed on April 19 and sat in an out-tray through an entire weekend while the green water spread from the Lea into the Thames, into the treatment plants, into the drinking supply of two million Londoners.

The phone rang in David's empty office. It was Saturday. David was at his daughter's flat in Camden, enduring a stilted conversation about her new boyfriend, a man named Rupert who worked in "finance" — which, in 1985, in the aftermath of the Big Bang deregulation, could mean anything from legitimate stockbroking to money laundering for the IRA. David did not care. He was thinking about the river. He was thinking about the green.

The network broke on April 26.

David Prentiss was sacked. The official reason was "redundancy following departmental restructuring," which was the Thatcher government's euphemism for everything. The real reason was that his report — the one about the river, the green, the something moving beneath the surface — had been leaked to a journalist at the Hackney Gazette, and the journalist had asked questions, and questions were not tolerated in the spring of 1985, when the government was still fighting the miners and the unions and anyone else who dared suggest that profit was not the highest good.

David cleaned out his desk. He took his rubber plant, his mug that said WORLD'S OKAYEST DAD, and a manila folder full of water quality reports that he was technically not supposed to remove from the premises. He walked out of the council offices on Bethnal Green Road and did not look back.

Artie Tibbs found the body on April 27.

He was walking the dock edge, as he did every morning, when he saw something floating in the green water. It was not human — not quite. It was a dog, a large Alsatian that had apparently fallen into the Lea somewhere upstream and drifted down to Bow Creek. The dog's fur was matted with green slime. Its eyes were open and filmed over with the same vivid, unnatural emerald that had consumed the river. And something was growing out of its mouth — a thin, hair-like tendril that swayed in the current as if it were still alive.

Artie did not call anyone. He had lived in the East End his entire life, through the Blitz and the smog and the industrial collapse, and he knew that calling the authorities was the fastest way to make a problem disappear without actually solving it. Instead, he went to find Jimmy Chen.

Jimmy was not at home. Jimmy had been taken to the Royal London Hospital three days earlier with what the doctors were calling "a severe respiratory infection." His lungs were filling with fluid. The fluid was green.

Toxin found the activist on April 28.

She was walking back from a squatting rights meeting at the Morning Star pub when she saw a woman in her forties nailing a poster to the wall of the closed-down printworks on Mare Street. The poster said: THE RIVER IS POISONED. YOUR WATER IS POISONED. THE GOVERNMENT KNOWS. The woman had the look of someone who had not slept in several days. Her name was Marjorie Khan, and she was a founding member of the London Green Alliance, a small environmental group that had been trying to get anyone to listen for three years.

"Did you know about the green?" Toxin asked.

Marjorie turned. Her eyes were red-rimmed. "I've been trying to tell people for a week. Nobody cares. Nobody bloody cares."

"I care," Toxin said, and was surprised to find that she meant it.

That evening, Toxin and Marjorie sat in Toxin's squat and compared notes. Toxin had seen the green water but had not understood what it meant. Marjorie had been tracking the algae's spread through samples, contacts, and the whispered warnings of a Thames Water engineer named Eleanor Hartley who had been placed on "administrative leave" after asking too many questions. Together, their knowledge formed a picture that neither could see alone. The algae had been engineered. It was spreading through the water supply. It was targeting humans — causing respiratory illness, organ failure, and a dozen other symptoms that the press was attributing to a "mystery flu." And the government, the water authority, and the chemical company that owned the factory upstream — a subsidiary of a subsidiary of a multinational that had been quietly dumping waste into the Lea for thirty years — were all working together to suppress the truth.

Nadia Begum stopped drinking the water on April 29.

The women in the factory had been getting sick for a week. Coughs first, then fevers, then something worse — a greenish tinge to the whites of their eyes, a greenish tint to their spit. Two women had collapsed at their stations. Mr. Chowdhury had dragged them into the corridor and called an ambulance that never came.

Nadia had started boiling the water from the taps. She did not know if boiling helped, but her grandmother in Sylhet had boiled water during the cholera outbreaks, and tradition was all she had. She was saving the factory women one kettle at a time, and she did not know that the source of the contamination was not bacteria that could be killed by heat but something far more insidious: a biological agent designed to survive, to spread, to avenge.

The chemical factory was called Petromark Industries. It had opened in 1952, on a site that had previously been a munitions works, which had previously been a tannery, which had previously been open marshland where the Lea ran clear and the eels were so thick you could scoop them out with your hands. Petromark produced industrial solvents, the kind used to clean machinery and strip paint and dissolve things that were not meant to be dissolved. For thirty-three years, Petromark had been discharging its waste into the Lea through a pipe that the water authority had approved in 1953 and never inspected again.

The pipe had not been designed to last thirty-three years. It had corroded. The solvents had seeped into the riverbed, into the groundwater, into the very soil of Hackney Wick. And somewhere in that toxic soup, over decades of exposure to heavy metals and chlorinated hydrocarbons and God knew what else, the algae had evolved. Or been changed. Or been pushed, deliberately, over the threshold that separates simple organism from avenging angel.

Nature holds a ledger of every harm. The Lea had been keeping its accounts for a very long time.

On April 30, the network reformed.

Eleanor Hartley, suspended but not silenced, drove to Hackney in her own car — a battered Austin Metro that smelled of river mud and coffee — and knocked on the door of Toxin's squat. She had tracked Toxin down through Marjorie Khan, who had been distributing leaflets outside the Royal London Hospital where Jimmy Chen lay dying. The leaflets had Toxin's address on them. Eleanor had recognised the address. It was the Somerford Grove Estate, where the water pressure had dropped two days earlier because the treatment plant was struggling to filter the algae out of the intake.

Eleanor told Toxin what she knew: the algae was not natural. It had been engineered somewhere, somehow, by something — she did not know if it was human intervention or a natural mutation accelerated by the chemical soup of the Lea, but it was deliberate in its function. It produced a neurotoxin that caused respiratory paralysis. It reproduced in the human bloodstream. It was, effectively, a biological weapon that had been growing in London's water supply for who knew how long, waiting for the right conditions to bloom.

Toxin told Eleanor what she had learned from Marjorie: the factory was Petromark. The discharge pipe was still active. The council knew about it and had known for years. David Prentiss had been fired for writing a report about it. Artie Tibbs, the old docker who lived on the narrowboat by Bow Creek, had been keeping a diary of the river's decline since 1979 and had documented every change, every fish kill, every strange colour in the water. And Nadia Begum, the garment worker — Toxin did not know her name yet, only that there was a woman in the Apex factory who had been boiling water for her colleagues, and who had sent one of the younger girls to the doctor with a sample of the green tap water, and the doctor had called the Health Authority, and the Health Authority had done nothing.

Eleanor drove back to Reading and broke into her own laboratory.

She was caught, of course. The security guard who worked the night shift — a retired policeman named Reg who had been perfectly happy to ignore her late-night work for years — found her at three in the morning, hunched over a gas chromatograph, running analyses that she was no longer authorised to run. Reg could have looked the other way. He had done so many times before. But the management had circulated a memo, and the memo had Eleanor's photograph on it, and Reg had a pension to think about.

"Sorry, love," Reg said. "I've got to call this in."

Eleanor was arrested for trespassing and theft of laboratory materials. The charges were dropped within forty-eight hours, but by then it did not matter. The algae had entered the drinking water of the entire East End.

Nadia's women started dying on May 2.

Two of them, first — the youngest ones, barely sixteen, who had been in England less than a year and whose immune systems had been weakened by poor nutrition and overwork and the bone-deep exhaustion of fourteen-hour shifts in an unventilated factory. The green had spread inside them as it had spread in the river — slowly, imperceptibly, until the moment it became visible, and by then it was too late.

Nadia stood outside the factory gates as the bodies were carried out. She had been boiling water for nothing. The factory's supply came from the Lea treatment plant, and the treatment plant's filters had failed, and the council had known about the failure for four days but had not issued a warning because to issue a warning would be to admit liability, and to admit liability would be to pay compensation, and Margaret Thatcher's government did not believe in compensation.

Artie Tibbs saw the television news on May 3 and finally understood what Jimmy had been trying to tell him. Canton, 1893. The green water. Fifty thousand dead. Jimmy was already unconscious in the Royal London by then, his lungs so full of algae that the doctors could hear it sloshing when he breathed, and Artie did what he should have done days earlier: he went to the police.

The police station on Bethnal Green Road had been rebuilt after the IRA bombed it in 1982. The new building was all concrete and reinforced glass, designed to withstand anything except the truth. The desk sergeant — a young man with the kind of moustache that had been fashionable in the late 1970s and now just looked sad — listened to Artie's story about the green water and the dead dog and the Chinese man dying in the hospital, and then he said, in the patient tone of a man who had heard a thousand conspiracy theories, "I'll make a note of it, sir. Someone will follow up."

No one followed up.

The network's final node clicked into place on May 4, in the curry house on Brick Lane where Toxin was eating a vegetable samosa with Marjorie Khan and David Prentiss — the sacked environmental officer, who had tracked them down after seeing Marjorie's leaflets and realising that he was not the only one who had seen the pattern. David had brought his manila folder of stolen reports. Marjorie had brought her contacts list — every journalist, every sympathetic MP, every community organiser she had cultivated in three years of thankless activism. Toxin had brought the address of the factory and the name of the garment worker who had been boiling water for her colleagues. Eleanor Hartley could not be there because she was in police custody. But her research notes had been photocopied and distributed before her arrest, and David had a copy, and the copy was sitting on the table between them, its pages stained with curry oil and desperation.

"We need to go public," David said. "Now. Tonight. Before anyone else dies."

"The papers won't run it," Marjorie said. "I've tried. The Hackney Gazette's editor told me they've been 'advised' not to publish anything about the water. The nationals don't care about Hackney."

"The television, then."

"The BBC is government-funded. ITV depends on advertising. Who's going to advertise on a programme that tells people their water is poison?"

Toxin lit a cigarette — Silk Cut, the brand her mother had smoked before lung cancer took her at forty-seven — and stared at the green algae sample that David had brought in a jam jar. It was still alive, still growing, still pulsing with that strange, arrhythmic heartbeat. She had been a punk squatter for two years, and in that time she had learned exactly one thing that was useful: institutions could not be trusted, but people could.

"Give me the file," she said.

That night, Toxin broke into the BBC's Lime Grove Studios.

She did not get very far. The security guards caught her in the lobby, clutching the file and screaming about green water and dead factory girls, and she was arrested and charged with trespassing and criminal damage and probably would have been sectioned under the Mental Health Act if not for the fact that the desk sergeant who processed her — the same man who had dismissed Artie Tibbs two days earlier — had begun to notice that his own tap water was running slightly green.

The story broke on May 6, not through Toxin's failed break-in, but through the cumulative weight of a network that had finally found its new centre. David Prentiss, unemployed but not silenced, called every number in Marjorie's book. Marjorie, exhausted but not defeated, stood outside the Royal London Hospital with a placard that said YOUR WATER IS KILLING YOU. Artie Tibbs, the old docker who had been watching the river for six years, walked into the offices of The Guardian and refused to leave until someone listened to his diary. And Nadia Begum, the garment worker from Sylhet, went to the Hackney Gazette with a jam jar full of green water and a photograph of the two sixteen-year-old girls who had died at their sewing machines.

The paper ran the story on the front page.

The government denied everything, of course. Petromark Industries issued a statement expressing "concern" and "commitment to environmental responsibility." The water authority claimed the algae was a "naturally occurring phenomenon" unconnected to any industrial discharge. Thames Water said the treatment plants were "functioning within acceptable parameters." And Margaret Thatcher, on the evening news, said that there was "no cause for alarm" and that "the government is monitoring the situation closely."

But the network had done its work. The story was out. The green could not be unseen. The water could not be undrunk. And the Lea, ancient and patient and holding its ledger of every harm, continued its slow, inexorable revenge.

Jimmy Chen died on May 9, the twenty-seventh recorded death from what the press had begun calling "Lea Sickness." His last words, whispered to a nurse who did not speak Cantonese, were the same words his grandfather had spoken in 1893: "The green remembers."

Artie Tibbs, standing at his usual spot on the derelict dock, watched the river and understood what Jimmy had meant. Nature was not a passive victim. Nature was a creditor. And the debt, long overdue, was finally being called in.

The green algae of the Lea had not appeared by accident. Something in the poisoned water, in the decades of chemical abuse, had triggered a response — not intelligent in any human sense, but purposeful. The algae was a weapon, grown and deployed by an ecosystem pushed past its breaking point. The river was fighting back. And the river, unlike the council, unlike the government, unlike every institution that had failed, did not forget and did not forgive.

On May 11, the Petromark factory was raided by environmental health officers who had finally, under public pressure, been authorised to do their jobs. They found the discharge pipe, still active, still pumping solvents into the Lea. They found records going back to 1963 documenting every discharge, every violation, every warning that the company had ignored. They found a memo from 1977, addressed to the managing director, that said: "Continued discharge into the Lea may result in unforeseen environmental consequences. Recommend relocation of outflow to the Thames estuary where dilution will be greater."

The managing director had written in the margin: "Too expensive. Continue as before."

The green had been foreseen. The green had been warned of. The green had been chosen — by men in boardrooms who valued cost savings over the lives of garment workers and squatters and old Chinese men who remembered Canton.

Nature, Nadia Begum learned, kept better records than any company. The algae had not mutated randomly. It had been pushed, shaped, selected by thirty years of chemical pressure until it became something that could fight back. The lagoon in Sylhet where Nadia had played as a child had been drained and filled for a textile factory. The fish her father had caught in the Surma River were gone, replaced by nothing but brown foam and the smell of dye. The world was full of Lea Rivers. The world was full of green water. And the reckoning, Nadia understood with the clarity that comes when the centre of a network fails and reforms around a new truth, had only just begun.

In her squat on the Somerford Grove Estate, Toxin — Emma Coulson, who would never be just Emma Coulson again — painted a mural on the wall of the abandoned flat. It showed the Lea as a serpent, winding through the East End, green and luminous and alive. In its coils were the names of the dead. Above it, in letters she had learned to form beautifully in the art school her mother had paid for before the cancer came, she wrote: THE RIVER WAS HERE FIRST. THE RIVER WILL BE HERE LAST. EVERYTHING IN BETWEEN IS BORROWED.

And in his narrowboat on Bow Creek, Artie Tibbs poured a cup of tea — boiled, double-strength, the way they made it in the docks — and raised it to the memory of Jimmy Chen, and to the river, and to the ledger that was not yet balanced.

The green receded eventually. It took six months and the demolition of the Petromark factory and the installation of new filtration systems that cost the government seventeen million pounds. The death toll stopped at thirty-four. The newspapers moved on to other stories. The East End, resilient as it had been through the Blitz and the slums and the industrial collapse, absorbed the tragedy into its long memory of tragedies.

But nobody who had seen the water as it was in April 1985 — vivid, lurid, pulsing with something that was neither life nor death but a reckoning between the two — ever drank from the tap again without looking first.