The Silent Patient Site

Timeline A (The Affair): Theo discovers his wife, Kathy, is cheating on him. He tracks down the man she is sleeping with—it is Gabriel Berenson. Theo breaks into their house to confront Gabriel. He holds Gabriel at gunpoint and gives him an ultimatum: "Who do you choose? Your life or hers?" Gabriel chooses his own life, proving he doesn't truly love Alicia. Theo leaves, but this encounter breaks Alicia. Realizing her husband does not love her, she kills him. Theo is the "stranger" who broke into the house.
Timeline B (The Treatment): Years later, Theo applies for a job at The Grove specifically to treat Alicia. He isn't trying to "save" her; he is trying to silence her. If she speaks, she will reveal that a masked man (Theo) was in the house that night.