Someone Behind the Door (1971)

Dr. Laurence Jeffries, a neurosurgeon, intent on his research, neglects his wife Frances. The frustrated woman accordingly takes a lover, Paul Damien, a French journalist. When he realizes what is going on, Laurence has only one thing in mind, vengeance. A diabolical idea comes to him: why not use one of his patients, an amnesiac, as his instrument of revenge? He soon implements his plan, taking the amnesiac into his home and making him believe that Frances is HIS wife, that she cheats on him and that he has to eliminate her...