The Argyle family is far from pleased to discover one of its number has been posthumously pardoned for murder - if Jack Argyle didn't kill his mother, who did?
Throughout his trial Jack had maintained his innocence, claiming he was hitchhiking on the night of the murder and he had been picked up by a middle-aged man in a dark car. Unable to locate this mystery man, the police viewed Jack's story as a lie.
Dr Arthur Calgary was the stranger in question, but he arrives too late for Jack - who succumbs to pneumonia after serving just six months of his sentence.
Feeling a sense of duty to the Argyles, Calgary is surprised when his revelation has a disturbing effect on the family - it means one of the family is a murderer . . .