Teenager Jan-Erik Widgren (Alf Kjellin) meets the lonely Bertha Olsson (Mai Zetterling), a woman struggling with alcoholism. Though Bertha is already romantically linked to Caligula (Stig Järrel), Jan-Erik's heartless teacher, she begins a relationship with the boy anyway. When Caligula learns that Jan-Erik is having an affair with Bertha, he begins to torture his student psychologically. He reserves his cruelest behavior for Bertha, however, which results in a tragic turn of events.
