Strangers on a Train (1951)

Two strangers meet on a train. They’ve never met before. Both of whom have someone they’d like to murder. So, they swap murders. A psychopath shares this concept with tennis star Guy Haines, whose wife refuses to get a divorce. He agrees, thinking it is a joke. But now his wife is dead, Haines finds himself a prime suspect and the man wants Guy to kill his father.