Horace Danby is a fifty-year-old unmarried lockmaker who is widely regarded as a good, honest citizen. However, he has a secret: he is not completely honest. To fund his expensive passion for collecting rare and valuable books, he robs one safe every single year. He plans each heist meticulously to ensure he steals just enough to last him twelve months.
Horace targets a house at Shotover Grange, which contains fifteen thousand pounds' worth of jewels. He spends two weeks studying the house's layout, wiring, and paths. When the servants leave for the movies, Horace breaks in, carrying his tools. However, his break-in is hindered by a severe attack of hay fever triggered by a bowl of flowers in the drawing room.
While sneezing, Horace is confronted by a young, pretty woman dressed in red who claims to be the wife of the house's owner. She acts calm, confident, and treats Horace with an amused authority. She threatens to call the police unless Horace helps her. She claims she forgot the combination to the safe and needs her jewels for a party that night.
Eager to please her and secure his freedom, Horace takes off his gloves and cracks the safe for her. He hands over the jewels and leaves happily. Two days later, Horace is arrested by the police. His fingerprints are found all over the safe because he opened it without gloves. It is revealed that the young woman was actually a clever thief herself; the real wife of the owner is a sixty-year-old, sharp-tongued woman who dismisses Horace's story as nonsense.
The story explores the classic proverb honour among thieves. Horace believes in this unspoken code of conduct, which leads him to trust the young woman. However, she exploits this trust to save herself and frame him, proving that there is no true honor in their profession.
Both main characters project false identities. Horace appears to be a respectable citizen but is a thief. The young woman appears to be the elegant owner of the house but is actually a cunning burglar. Horace's failure to see past her appearance leads to his downfall.