The novel predominantly takes place in the slums of Montreal, Canada. Baby, the thirteen-year-old protagonist of the novel, lives with her heroin-addicted father, Jules. Her mother died when she was still a baby, leaving fifteen-year-old Jules to care for her on his own. His young age, addiction, and overall immaturity leads to an unstable and dangerous life for Baby.

The novel is told from Baby’s adult point of view, and each section and subsequent chapters are told as memories from her childhood. This results in a confessional narrative, where Baby is looking back on her life but is still maintaining a childlike honesty within her memories. Each section has a title that situates the specific memory, and within each section there are numbered chapters that allow the reader to follow along linearly. These sections become vignettes that connect temporally but can also stand alone.

When the novel begins, Baby is twelve and still very innocent. Despite her dad’s heroin addiction and continual negligence, in her eyes he can do no wrong. Only when Jules is taken to a hospital for tuberculosis and she is put into a foster home does she realize that she craves stability and maternal affection; she also likes connecting to other children her age, most notably Linus. When Jules gets better, Baby leaves the warmth of the foster home and moves in with him. He’s still using heroin, and although she’s happy to be reunited with her dad, she’s now aware that he can’t really be the dad she wants him to be because of his addiction.

Jules gets arrested for heroin and is sent to a rehabilitation center. Baby goes to live with her neighbor, Mary, who has two sons: Johnny, who’s eighteen and inappropriate with Baby, and Felix, who’s Baby’s age. She grows attached to her life with this family, but soon Jules is out of rehab and takes her back. He’s not on heroin anymore, but now he treats Baby differently. He’s mean and argumentative, and she feels like he doesn’t love her anymore. She joins a community center to feel connected, but eventually Jules makes her quit. Feeling lonely, she befriends an abusive boy named Theo, until he is sent away to a foster home.

When Baby turns thirteen, she and Jules are more distant than ever—by this point, Jules is having paranoid behavior and spending much of his time on the streets. At one point, someone calls him schizophrenic, although this diagnosis is never confirmed in the novel. Baby feels alone, neglected, and desperate for attention, so when Alphonse, the attractive neighborhood pimp, gives her some, she soaks it up. Eventually, he sleeps with and prostitutes her. He takes all the money she earns and tries to give her rules. She becomes resentful and starts dating Xavier, a boy at her school. The novel juxtaposes her innocent love for Xavier with the adult men Alphonse forces her to have sex with.

Towards the end of the novel, Baby is living with Alphonse, addicted to heroin, and hasn’t seen Jules in nearly two months; she is still only thirteen. The climax occurs when she is high on heroin and invites Xavier to the hotel room she shares with Alphonse. She and Xavier sleep together, and Alphonse returns. He beats up Xavier and takes Baby’s heroin. He shoots up the heroin and dies, and Baby finds her dad at the homeless shelter. Jules is overjoyed to see her, and the last scene of the novel is of him and her taking a bus to the country to stay with his cousin, Janine.