Piranesi

The mystery of the House resolves once Piranesi reconstructs the destroyed journal entries he finds hidden in gull nests. Piecing the scraps together, he learns his own forgotten history: he was once Matthew Rose Sorensen, a journalist researching the occultist Laurence Arne-Sayles (the man Piranesi has been calling the Prophet). When Rose Sorensen went to interview one of Arne-Sayles's former students, a man named Ketterley (the Other), Ketterley used a ritual to trap him inside the House. Over the following years the House eroded his memory and identity until he became, in his own mind, simply Piranesi.

Meanwhile, the sixteenth person Ketterley warned Piranesi to avoid turns out to be a woman named Raphael, a British police detective who has been investigating the string of disappearances connected to Arne-Sayles. Arne-Sayles, out of spite toward his former pupil, gave her the rite needed to enter the House. On the day a rare, catastrophic confluence of tides is due to flood the middle level, Piranesi confronts Ketterley with his restored memories just as Raphael arrives. Ketterley, desperate to keep his secret, tries to kill them both, but he is caught and drowned by the rising floodwaters himself.

Once the water recedes, Raphael explains the full situation to Piranesi and tells him that his family has spent all six years he was missing mourning him as dead. She urges him to return with her to his old life in the modern world. After much deliberation, weighing his attachment to the House and to the identity he built there against the pull of his former life and the people who loved him, he decides to leave.

In the epilogue, the narrator has resettled in the ordinary world but does not sever his connection to the House entirely; he returns to it periodically, continuing to care for the bodies of the dead there, including Ketterley's, just as he once tended the other thirteen skeletons he catalogued. He permits James Ritter, the earlier victim of Arne-Sayles who was also once trapped in the House, and Raphael to visit the House with him from time to time. He recognizes that he is no longer wholly Matthew Rose Sorensen nor wholly Piranesi, and that he must build a third identity out of the remnants of both. The book closes with him coming to see the modern world itself as an extension of the House's strange divinity, as he begins to notice the real-world counterparts of the House's statues appearing in the people and scenes of his everyday life.