Richard Marx plays this guy accused of murder but he maintains is innocence. In the beginning he is shown being teased as a child. Later he finds out his mom is an adulterer and burns a random person's house as a little kid. He eventually dates Mary, who is found dead in the river. It is up to the listener to decide if Marx's character killed Mary or not.
I'd like to think he did it because an easy listening song by a murderer is funnier than a power ballad sung by a cold blooded killer.