The book is a mental portrait of a damaged Irish youth's descent into tragedy. It is told in the provincial vernacular of the protagonist, Francis Brady. His boyhood friend Joe Purcell grows out of the mischief and delinquency of their childhood, while the protagonist, "Francie," cannot escape the downward spiral it portends.

Francie and Purcell befriend the neighbor boy, Phillip Nugent, recently returned from an English boarding school, only to steal his comic books. The Nugents are of a higher social strata, and Francie finds himself longing for a stable mother like Mrs. Nugent, but when she responds to the theft by calling the Brady family "a bunch of pigs," his feelings turn to hatred. He admires his popular and jovial uncle Alo, only to witness him berated by his drunk father, after which Francie runs away to Dublin to become a petty thief. He returns with a present for his depressive and abused mother, vowing he will never let her down, only to find she has drowned herself. His father blames him for her death. He plans to attack Phillip Nugent by luring him into an ambush and then defecates on the Nugents' floor and is arrested.

Francie is sent to a correctional school, where he is subjected to sexual advances by a priest. Upon his release, Francie tries to re-establish his friendship with Joe Purcell, who is now away at school, but discovers that Joe is now friends with Phillip Nugent, and his overtures are rejected. He also visits the boarding house where his parents had honeymooned to discover his father had been an abusive drunk even then. Francie takes up drinking and fighting. In his inner dialogue, he vows never to let his father down, and he takes a job in a butcher's shop but is eventually arrested again and confined to a mental institution. Through this process we learn that he has been living for some time with his father's corpse. The reader by this point realizes that this is a portrait of mental illness.

After his release, Francie returns to his job, in the course of which he visits the Nugent house and murders and dismembers Mrs. Nugent and writes the word "pig" in her blood around their house. He is arrested and leads the police to her body.

Francie was unable to adjust to life with his own parents or his community. Critics have identified the theme of cultural anomie or lack of a healthy Irish identity under colonialism. Clues that this was the author's intent are found in the names of the characters. Purcell and Nugent are both Anglo-Norman names brought into Ireland with the first colonial conquest under Strongbow. They symbolize the foreign colonial occupation under which the Irish had to live and the separate cultural world to which many of the Irish could not adapt and came to both admire and resent. Mrs. Nugent and Purcell are both symbols of desire, rejection, and resentment in the novel.