In Akron, Ohio, 24-year-old Joy and her five-year-old son Jack live in a squalid shed they call Room. They share a bed, toilet, bathtub, television, and rudimentary kitchen; the only window is a skylight, all of which Jack calls by name, as if they were living. They are captives of a man they call Old Nick, Jack's biological father, who abducted Joy seven years prior, and routinely rapes her while Jack sleeps in the closet. She tries to stay optimistic for her son, but is suffering malnutrition and is sometimes overcome with depression. She allows Jack to believe that only Room and its contents are "real," and that the rest of the world exists only on television.

Old Nick tells Joy that he lost his job and threatens he may not be able to afford their supplies in the future. That night, Jack is overcome with curiosity and ventures out of the closet while Old Nick is asleep in bed with his mother. Joy is horrified when she awakens and sees their interaction, slapping Old Nick away. As punishment, he cuts their heat and power. Joy decides to tell Jack about the outside world; he reacts with disbelief and incomprehension, but also curiosity. She has Jack fake a fever, hoping that Old Nick will take him to a hospital where he can alert the authorities, but Old Nick says he will return the following day with antibiotics.

Joy wraps Jack in carpet and has him play dead in the hope that Old Nick will remove him from Room. Falling for the ruse, Old Nick places Jack in the back of his pickup truck and drives through a residential neighborhood. Although stunned by his first exposure to the outside world, Jack jumps from the truck and attracts the attention of a passer-by. The police arrive and rescue him. Joy is found, reunited with Jack and the two are taken to the hospital.

Reunited with her family, Joy learns her parents have divorced and that her mother has a new partner, Leo. She returns with Jack to her childhood home where her mother and Leo reside. Her father cannot accept Jack and leaves. Jack struggles to adjust to life in the larger world, speaking only to his mother and expressing a desire to return to Room. Joy struggles with anger and depression, lashing out at her mother and ignoring doctor's appointments.

At the suggestion of the family’s lawyer, Joy agrees to a television interview, but becomes angry when the interviewer questions her decision to keep the newborn Jack instead of asking Old Nick to drop him at a hospital. Overwhelmed with guilt, she attempts suicide; Jack finds her unconscious in the bathroom, and she is admitted to a hospital.

Jack misses his mother but begins to settle into his new life. He bonds with his new family, meets Leo's dog Seamus and makes friends with a boy his age. Believing his long hair will give Joy the strength she needs to recover, Jack has his grandmother cut it for him so he can send it to her. Joy returns home and apologizes, thanking Jack for saving her life again.