Someone Behind the Door (1971) on Netflix

Plot summary

The neurosurgeon Laurence Jeffries gives his attention to his work and neglects his wife Frances, who turns to Paul Damien, a French journalist. Once Jeffries understands the situation, he settles on revenge, and he finds a tool for it in one of his patients: a man who has lost both his memory and his name. Jeffries brings the man into the family home and works on him, teaching him that Frances is his own wife, that she is deceiving him, and that it falls to him to remove her. A man who remembers nothing, Jeffries reasons, will never link him to the act.