A neurotic young man, Pablo, is treated in a psychiatric hospital. He poisons Alberto who shares his bedroom. His victim comes from a wealthy family and Pablo envies him. Death is attributed to a heart attack. Usurping Alberto's name, Pablo comes back to the family home the real son deserted twenty years ago. The parents are dead. There's only a young sister, who was born after Alberto left the house. That's going to make things easier for the murderer. But he passionately falls in love with his false sister, Silvia. As he cannot marry her, he plans to kill her.