the fly (1958)

Helene Delambre (Patricia Owens) admits to François (Vincent Price) that she has killed her husband, and his brother Andre (Al Hedison). It turns out that she has crushed him in a press, rendering his head and one arm to mush. Inspector Charas (Herbert Marshall), called in to investigate, convinces Helene to tell him the circumstances that led up to the murder. She reveals a story where Andre had invented a matter transporter, and used himself as a guinea-pig. Leading to tragedy when unknown to him a fly enters the transporter, and they emerge with their heads and one arm exchanged. With Andre put out of his suffering she still has to find the fly, and convince people she is not mad!
A classic of the mad scientist boldly going where no man should go and getting punished for it genre. The film builds a nice picture of marital bliss, before shattering it with one of the all time classic shocker scenes.