A sinister yet charming criminal, Professor Marcus (Alec Guinness) rents a room in the flophouse run by a sweet, old widow Mrs. Louisa Wilberforce (Katie Johnson). Posing as musicians, the Professor and his four other con-men gang members are plotting a complex armored-car robbery. After the successful hold-up, Mrs. Wilberforce unknowingly helps the gang by escorting the loot past the police. But as the gang leaves her house, One-Round (Danny Green) spills his cello case stuffed with money in front of Mrs. Wilberforce, and she informs Marcus that she is going to call the police. Seemingly with no other alternative but to kill the widow, the gang seems impossibly inept in carrying out this murder as no one seems to want to kill the old woman. One by one, the gang members double-cross and kill each other to avoid the job of killing the lady.