Plot summary
Sam goes to confront his abusive father and nearly kills him, but stops himself. Back in the basement, Alan tries to turn that restraint into a reason for Sam to change. He tells Sam that choosing not to repeat his father's violence matters, and argues that their agreement should now end with Alan's release.
Sam will not let him go. Instead, he begins imagining a much longer arrangement, with Alan continuing to live as his prisoner. Alan realizes that the apparent progress has not made him safe or given him control over whether he can return to his family.
During their next session, Alan insists that Sam needs confinement and must surrender to the authorities. Sam refuses. Desperate to force a way out, Alan threatens Sam's mother. Sam reacts by killing Alan, ending the possibility that the therapist will survive his captivity.
Afterward, Alan's children receive the letter he wrote for them. Sam delivers Alan's final words rather than keeping his death entirely hidden from the family.
Sam then chooses a form of imprisonment for himself. He locks himself in the basement where he held Alan and gives his mother the key. The ending leaves her responsible for containing him. It is not an acquittal of what he has done or a miraculous cure: Alan is dead, his family must live with that loss, and Sam has finally accepted that he cannot simply trust himself to stop.
Credits
Adapted from Wikipedia contributors, The Patient. Series, exact episode number and English title matched. Paragraphs formatted; references and illustrations omitted. Finale rewritten in new prose for clear causality and outcome; no claim that Sam is cured.