The sea did not care. Conall O'Sullivan understood this on the third day, when the boat that had brought him to the islet failed to return because the weather turned, and he was left standing on the rocky shore watching the grey water swallow the boat the way a mouth swallows a word that should not have been said.

The lighthouse rose behind him — a white cylinder eight stories tall, its surface stained by salt and time. It had been decommissioned in 1878, when the coast of County Wicklow was equipped with modern steel towers and this old stone sentinel was no longer needed. But the base had been converted — carved into rooms and corridors and a small institution that specialized in "psychological disorders of a particularly refined and intractable nature."

Which was to say: it was a place where people came to be treated for the disease of knowing too much.

Conall had been invited by Lord Ashford, an Irish aristocrat with a passion for the bizarre and the resources to act on it. The invitation had been elegant: "I am funding a treatise on psychological medicine. Your reputation precedes you. Come to the islet and examine a case of unusual complexity."

Conall had come because he needed the work. He had been a lecturer in psychological medicine at the Royal College of Surgeons in Dublin until two years earlier, when a public demonstration of post-mortem anatomy had gone wrong. He had identified a cadaver incorrectly. The "corpse" he dissected in front of thirty witnesses had been alive — comatose, yes, but alive. The woman died on the table. His career ended before her body was cold.

His wife, Siobhan — the deceased woman's sister — threw herself into the sea at Howth Head on the day of the funeral.

Conall did not think about this often. He thought about it never. There was a difference. Thinking about it required acknowledging that he had a past, and acknowledging his past required acknowledging that his past contained a wo

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