Una Drake's unbreakable-alibi wager

Una Drake’s unbreakable-alibi wager is a detective puzzle built around the claim that one woman occupied two places at once. Its solution, that Una’s twin sister Vera went to Torquay while Una stayed in London, converts an apparently impossible alibi into a test of careful investigation—and gives Tuppence the decisive role in solving the case.

The Wager and Its Challenge

The case begins in conversation between Montgomery Jones and Una Drake, who share an enthusiasm for detective stories. Their discussion turns from a particular mystery to the practical question of whether an alibi can be faked. Jones considers it extremely difficult; Una disagrees, arguing that it requires only some brainwork. She then offers to produce an alibi nobody can undermine. Jones accepts the sporting challenge, and Una’s confidence makes the wager sound less like a boast than an invitation to find the trick.

Jones brings the puzzle to the Beresfords through the detective office. He gives Tommy a written account of the two competing versions of Una’s day: by one, she ate alone at the Bon Temps Restaurant in Soho, attended the Duke’s Theatre, and later supped with Mr. le Marchant at the Savoy; by the other, she was staying at the Castle Hotel in Torquay and did not return to London until the following morning. The task is not merely to verify sightings, but to establish which account belongs to Una and how the other could also appear true.

Testing the Accounts

Tuppence and Tommy check the claims rather than treating either itinerary as self-proving. Their investigation establishes that Rice really did see Una at a distance in Torquay. The sighting confirms that the Torquay account has a basis in fact, but it does not establish that Una herself was the person Rice saw. That distinction—between a reliable observation and an assumption about the identity of the observed woman—is central to the eventual answer.

At lunch in a Soho restaurant, the pair compare notes and begin setting their findings down in an orderly schedule, as detectives in stories might do. The contrast between the neatly listed appearances and the apparent impossibility of the timetable keeps the puzzle focused: the evidence need not be false for the conclusion drawn from it to be mistaken. Tuppence eventually announces that she has solved the mystery and treats the case as complete enough to justify charging Jones for their work.

The Twin-Sister Solution

Asked for the answer, Tuppence gives it in a single word: twins. Una has a twin sister, Vera, who arrived in England the previous Monday. Vera went to Torquay, while Una stayed in London. The separate journeys explain how witnesses could place the two women in different locations without requiring Una to have made an impossible trip or to have fabricated every element of her account.

Tuppence says she learned of Vera by cabling Australia and receiving the information she needed. The discovery reframes Una’s wager: the apparent feat was possible because the puzzle’s premise concealed a second person who could be mistaken for Una. Tuppence also connects that possibility to an earlier conversation with Tommy about mothers-in-law, twins, and bottles of beer, saying that it put the idea into her head. The solution thus depends not only on checking the sightings but on recognizing that the identity assumed in the two accounts may not be the same.

Outcome and Significance

The wager’s outcome carries a personal as well as investigative consequence. Tommy asks whether Una will be terribly disappointed to have lost. Tuppence thinks not, and expects that Una will give Jones the credit for the solution. Her closing comment—that respect for a husband’s abilities should be the foundation of married life—turns the case’s resolution into a playful reflection on marriage and shared credit, without changing the evidence that made the puzzle solvable.

As a compact detective episode, the case makes identity the decisive clue. A real sighting in Torquay initially seems to support an impossible itinerary; the twin discovery preserves the sighting while correcting the assumption that it was Una who had travelled there. The answer rewards the Beresfords’ comparison of claims and evidence, and gives Tuppence the final explanatory move: the alibi is not unbreakable because one woman was in two places, but because two sisters made the two accounts possible.