In Akron, Ohio, 24-year-old Joy Newsome 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. 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 suffers from malnutrition and 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 has lost his job and 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 plans to help Jack escape Room by telling Old Nick that Jack has died from his illness. She wraps Jack in a carpet and has him play dead, telling him to run to the first person he sees for help. 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 passerby. Old Nick attempts to drag Jack back to his truck, but fails in the process. He drops Jack and flees the scene. The police arrive and rescue Jack. The police discover Joy as well, and take her and Jack to a hospital. Old Nick is captured and arrested.