Strangers on a Train (1951) directed by Alfred Hitchcock
Two men meet accidentally on a train. Guy Haines (Farley Granger) is a minor tennis star planning a political career after tennis. Bruno Antony (Robert Walker) is a chatty socialite with lots of ideas but little execution. He feels held back by his father, a rich man who ought to be giving Bruno full freedom rather than expecting him to work. He's so frustrated, Bruno wishes his father was dead. Bruno has read about Guy and knows quite a bit--Guy is dating a senator's daughter but still hasn't finalized a divorce with his current wife, who has been running around on Guy. Guy demurs about whether he is angry enough to kill his wife. Bruno pulls out one of his ideas to solve both their problems--they should swap murders! Bruno could kill Guy's wife and he would never be suspected because he has no motive. Guy could kill Bruno's father and also be free and clear. Guy is clearly uncomfortable with the conversation but humors Bruno until Guy gets off the train. He assumes Bruno is kidding around.
But Bruno isn't--he kills the wife and then starts pestering Guy to keep "his end of the bargain." Guy has a slim alibi for his wife's murder and plenty of motive, so the cops are all over him. Guy can't tell the true story to the police because Bruno will just say that they planned it together. How can he get out of an ever worsening situation?
Hitchcock puts together a top-notch thriller. The premise is a bit far-fetched but it plays out fairly logically and viewers get swept up into Guy's predicament. He tries to keep his girlfriend and her family out of it but Bruno becomes more insistent and starts invading their lives as well. Walker gives a great performance as Bruno, at times goofily earnest and at other times intensely menacing. The murder takes place at an amusement park and the big finale comes back to the amusement park. Both scenes deliver lots of tension and excitement. While not his most famous movie, this one is still quintessential Hitchcock.