Frank Cauldhame lives with his father, Angus, on a small island in Scotland. They are easily connected to the mainland by a bridge, but Frank is nonetheless cut off from the outside world. Angus claims Frank has no birth certificate, and as a result, Frank has spent his entire life on the island in relative isolation—his father homeschooled him and addressed all of his medical concerns.
Frank’s brother, Eric, was hospitalized several years earlier after a mental breakdown that caused him to become violent and dangerous. The book opens with the news that Eric has just escaped from the hospital, and is making his way back home. The novel takes place over the course of several days, both in the present, as Frank prepares for Eric’s return, and in the past, as Frank provides glimpses at his and Eric’s lives, and how they came to be the way they are.
Frank believes that when he was three years old he was attacked and castrated by the family dog, Old Saul. His castration has shaped his entire life, causing him to feel unmanly and forcing him to overcompensate for this with murder and violence. His need to get back at Old Saul causes Frank to kill his little brother, Paul, who he sees as a reincarnation of the dog, and later provokes Eric, after his breakdown, to set dogs on fire as a kind of retribution against the long-deceased family pet.
In addition to Paul, Frank has murdered two other children. He killed his cousin Blyth, as revenge, after Blyth set Frank and Eric’s pet rabbits on fire. Frank also killed his cousin Esmerelda, not out of any particular dislike for her, but because he felt he had killed too many male children, and felt it essential to help restore balance to the world by killing a girl.
Although Frank hasn’t killed anyone in many years, he remains violent and superstitious. He has set up a series of Sacrifice Poles around the island that act as sentinels, and for most major decisions he looks to a contraption he has built called the Wasp Factory. The Factory is an old town clock that Frank has placed on the floor of his loft. He has attached twelve death traps to the twelve numerals. When he has a question, he will release a wasp into the Factory, and depending on how it dies, he draws a different conclusion.
Over the course of a week Eric calls Frank from locations across Scotland, coming closer and closer. He eventually arrives on the island to Frank’s excitement and dismay—he is happy to see his older brother, but he is also afraid of being on the receiving end of the destruction and violence he knows his brother is capable of.
On the same night Eric arrives, Frank discovers a secret about his past. Frank was actually born a girl named Frances, and was not castrated by Old Saul. In reality, Frances had never had any male genitalia. Although Frances was attacked by Old Saul as a child, the damage was minimal. Nonetheless, Angus took this as an opportunity to conduct an experiment on his child. Angus fed Frances male hormones, and raised Frances as a boy. Frances is shocked by this, by quickly comes to terms with this new identity, greeting Eric the next morning as his new sister.