The House is a magical world made up of endless hallways lined with unique statues placed on top of columns. It is divided into two levels; the upper level is covered in clouds and the lower level is ravaged with the ocean. Between the two levels is the middle level, which is prone to violent tidal waves from the ocean below. It is Piranesi's job to monitor and evaluate the tide levels.
The only other person in the House is known as the Other. They frequently meet up to look for a precious knowledge hidden in the House. After a long while, Piranesi suggests they give up the search, but The Other informs him that this is not the first time he has suggested this. It is revealed that living in the House for long periods of time erodes one's memory.
The Other informs Piranesi that another person known as sixteen will arrive at the House to attack him, and he must stay clear of them at all cost. Later, Piranesi meets an old man who claims that the Other is a fraud who stole his quest of the precious knowledge hidden in the House. Piranesi discovers an entry in one of his journals about a man named Laurence Sayles. He was an occultist and the Other was one of his students.
As Piranesi is routinely monitoring the tides, he discovers that an unusual amount of tide will soon hit the middle level. Going through a destroyed journal, now restored, Piranesi discovers that his name used to be Matthew Sorensen, a journalist who was writing a book on the cult following of Laurence Sayles. The Other now revealed as Ketterley kidnapped and imprisoned him in the House where he lost all his memories over time.
Piranesi confronts Ketterley, who tries to kill him but is killed by the incoming floodwater. It is revealed that Sixteen is a detective named Raphael. She had been investigating Sayles' cult and the disappearances surrounding it. Piranesi leaves the House and goes back to his home world.
