The Unexpected Guest by Agatha Christie

Lost in the fog, a stranger seeks refuge in a nearby house only to find a man shot dead and his wife standing over him with a smoking gun. But the worman’s dazed confession is anything but convincing and the unexpected guest decides to help. As the ghosts of a past wrong begin to emerge, a tangled web of lies reveals family secrets and chilling motives, where the real murderer turns out to be the greatest mystery of all.