The Prophet claims that the House is a 'distributary world', formed by ideas flowing out of another world. Piranesi meets an elderly stranger he calls the Prophet, who identifies the Other as Ketterley, a rival who stole his ideas about the Knowledge. The Other warns Piranesi that a sixteenth person, whom both call '16', may enter the House to do him harm, and that he must not approach 16 under any circumstances or he will lose his sanity. When Piranesi suggests that they abandon the quest for the Great and Secret Knowledge, the Other says they have had this conversation before, and warns Piranesi that the House slowly erodes one's memories and personality. The Other occasionally brings Piranesi supplies that seem to originate from outside the House, such as shoes, electric torches, and multivitamins. Twice a week, Piranesi meets with the Other, a well-dressed man who enlists his help to search for a 'Great and Secret Knowledge' hidden somewhere in the House.

Piranesi records every day in his journals, the text of which makes up the novel. He believes he has always lived in the House, and that there are only fifteen people in the world, most of whom are long-dead skeletons. The upper level of the House is filled with clouds, and the lower level with an ocean, which occasionally surges into the middle level following tidal patterns that Piranesi meticulously tracks. Piranesi lives in a place called the House, a world composed of infinite halls and vestibules lined with statues, no two of which are alike. Piranesi won the 2021 Women's Prize for Fiction.

The story is told through the research notes of the eponymous narrator, who reconstructs the story of his own arrival as he explores this world. The novel is set in a parallel universe made up of an infinite number of halls and vestibules, which triggers a gradual loss of memory and identity in newcomers. It is Clarke's second novel, following her debut Jonathan Strange & Mr Norrell (2004), published sixteen years earlier. Piranesi is a fantasy novel by English author Susanna Clarke, published by Bloomsbury Publishing in 2020.

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