If I Stay begins with seventeen-year-old cellist Mia Hall waking to a thin layer of snow on her lawn. In Oregon, this small amount of snow is enough to cause school closures. Mia's brother Teddy and her father, a middle-school English teacher, also have the day off, so her mother takes the day off of work. The family pile into their rusty Buick, planning to go out for a day trip to visit friends and family.
A road collision kills Mia's mother and father. After the accident, Mia's body slips into a coma, while her consciousness is able to observe the scene in an out-of-body experience. She feels no pain, and wonders if she is dead.
Paramedics rush Mia and Teddy to the local hospital, where Mia sees her parents' friend Willow on shift as a nurse. Mia is airlifted by helicopter to a hospital in Portland. While her body is in the intensive care unit, Mia observes her grandparents, her best friend Kim, and her boyfriend Adam as they wait and worry in the hospital. Meanwhile, Mia reflects on memories of her family and friends. She recently applied to Juilliard music school in New York City, which added tension to her and Adam's relationship.
Mia overhears her grandparents discuss whose responsibility it is to decide whether Mia lives or dies. Mia realizes the choice is hers. Later, Mia learns that Teddy has died. She reflects on the pain it would cause her to live without her family and decides she would prefer not to wake from her coma.
In the book's final scene, Adam places headphones over Mia's ears and plays a recording of Yo-Yo Ma, her favorite cellist. Listening to the music, Mia considers the possibility of a bright future while simultaneously grieving for what she has lost. The sensations of grief and optimism overwhelm her until she returns to her body and squeezes Adam's hand. She wakes from her coma to hear Adam saying her name.