Lancelot is a novel by critically-acclaimed writer Walker Percy. Lancelot is a cuckolded man who finds himself locked up in a mental institution in the aftermath of a fire at his home, Belle Isle. Lancelot claims not to remember the circumstances that led up to his confinement until he sees an old friend, Harry, in the hallway. Lancelot shares his story with Harry a little at a time, showing the reader how Lancelot went from a mild-mannered, if bored, husband, to a murderer. Lancelot is a fascinating tale of madness that takes the reader into a world that could all too easily belong to anyone.

Lancelot welcomes Harry, the priest-psychiatrist, into his room at the mental institution, expressing confusion over whether or not he is a patient or a prisoner. Lancelot claims not to have known what circumstances brought him to this place until he saw his old childhood friend in the hallway one afternoon. Now Lancelot has chosen to tell his story only to Harry. Lancelot begins by telling Harry how he came to realize that his wife was cheating on him. Lancelot was in his study, a converted pigeonnier, where he was reading a novel. Lancelot noticed an application for camp on his desk for his daughter that his wife wanted him to sign. Lancelot glanced the form over and saw that it had his daughter, Siobhan's, blood type listed. Lancelot realized almost immediately that the blood type listed was incompatible with his own. Lancelot called a friend, a doctor, and confirmed that his child would not have that blood type no matter what blood type the mother might be.

Lancelot then checked his files and realized that his wife, Margot, was in Texas at an acting seminar about the time she became pregnant, suggesting that Merlin, an actor, was the father. Ironically, Merlin and group of actors were staying at the house at that time making a new film at Belle Isle. Lancelot began watching this group of actors, attempting to find proof that his wife was having an affair. Lancelot stopped drinking and found himself moving out of the rut he has been in since his marriage began. At this point, Lancelot stops to talk with Harry about love and its meaning, discussing briefly his first wife, Lucy. Lancelot also talks with Harry about how he met Margot when she was working as a tour guide at his home and how her fascination with Belle Isle might have led to her fascination for Lancelot.

Lancelot returns to his story, telling Harry how he enlisted the help of his house servant, Elgin. Lancelot tells Harry that he sent Elgin to the hotel where the movie people were staying and had him spy on them. Unfortunately, Elgin could not get enough information to prove Margot was sleeping with any of the men associated with the movie. Lancelot then asked the owner of the hotel, a cousin, to close the hotel until after a pending hurricane moved on. This caused the movie people to move back into the house where they had been invited to stay as guests. Lancelot then had Elgin place hidden cameras in each of the guest rooms so that he could record their activities. These cameras caught proof of Margot's unfaithfulness as well as the corruption of Lancelot's eldest daughter, Lucy.

Lancelot tells Harry that he hopes to get out of the hospital soon. Lancelot will begin a new world with the woman in the room next to his, a new world in which women will be clear about the sexual identity, whether they be virtuous women or immoral women. Lancelot also plans to take custody of his young daughter, Siobhan, despite his conviction that she is not his biological daughter. When Lancelot returns to the story, he tells Harry how he piped gas from the basement of Belle Isle into the guest bedrooms. Lancelot then went into each of the bedrooms to make sure his victims were present. Troy Dana was passed out on the bed, but this did not stop his lover from seducing Lancelot and making love to him next to Troy. In Margot's room, Lancelot walked in on Margot and Janos Jacoby in the middle of love making. Lancelot climbed into the bed with them, provoking a fight. As the fight came to an end, Lancelot slit Janos' throat. Moments later, the gas ignited and blew Lancelot out into the front yard. Lancelot returned to retrieve the knife, sustaining the burns that were attributed to his attempts to save his wife's life in the press.