The Unexpected Guest - Agatha Christie
On a foggy night, a man called Michael Starkwedder breaks down near an isolated house and, entering it, finds the body of a dead man slumped in a chair. A woman stands over the corpse, gun in hand, and confesses to the murder. She gives her name to be Laura Warwick, the wife of the dead man. She explains that he was abusive, and always had his drink. Michael decides not to turn her in to the police, and the two decide to come up with a cover-up story to protect Laura. In the end, the settle on an enemy from the past, by the name of MacGreggor. His son was run over by Richard Warwick, the dead man, several years ago. They slip a paper in Richard's pocket with the date of the accident, saying "Paid in full." Then, they stage the murder so it appears to have been recent, alerting the residents of the building.
The police are soon alerted, and begin to investigate. It is revealed the MacGreggor is dead, and suspicions are exchanged. Meanwhile, Michael discovers the Laura was having an affair with another man, who she believes to have murdered Richard. He, however, believes her to be guilty. Finally, it is revealed that Michael is MacGreggor, and he had come to avenge his son. He shouts this to Laura, along with the fact that he cares for her, and jumps through the window, running away.