Piranesi discovers that 16 has entered the House, and leaves a message. Ritter later described being held captive in a place resembling the House. Arne-Sayles fostered a cult-like mentality among his followers and was eventually imprisoned for kidnapping a man named James Ritter. The entries tell the story of an occultist from the modern world named Laurence Arne-Sayles who posited that other worlds existed and could be accessed Ketterley was one of his students. While indexing his journals, Piranesi discovers references to entries he doesn't remember writing which include terms mentioned by the Prophet. He declares he will lead 16 to the House in order to hurt Ketterley. 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 hardcover