Steven Murphy (Colin Farrell), a talented cardiologist, finishes a surgery and then goes to a restaurant where he meets a teenager named Martin (Barry Keoghan) who appears to have a form of mental illness. Steven seems to be a true mentor to Martin, but the exact nature of his relationship is unspecified. Afterwards, Steven returns to his wife Anna (Nicole Kidman) and their children, Kim (Raffey Cassidy) and Bob (Sunny Suljic), telling them about Martin and being a college student with Kim.
Later, he tells Anna that the boy’s father died in a car accident ten years ago and wants to help the boy overcome the trauma. At Steven’s suggestion, Martin comes to their home to have dinner together, and Kim is friends with him. After lunch, Steven, who is obviously uncomfortable with the situation, is trying to leave, but Martin insists he will still see a movie with them. Martin goes to the middle of the movie, and his mother makes some advances to Steven who rejects them and leaves.
Over the next few days, Martin’s need to spend time with Steven becomes more and more desperate, but Steven is no longer responding to requests. Everything seems to be fine until one morning Bob wakes up and realizes that he can not feel his feet anymore, becoming virtually paralyzed. Steven and Anna take him to the hospital where a neurological examination reveals that the boy has nothing physical. Even though he gets back, Bob still can not walk on his own feet.