Crime has gone down in London. It leaves Tommy and Tuppence with little to do. Tuppence suggests that they go to Paris to investigate lost paintings as there is a reward for its safe return.
Monty Jones turns up with an unusual case. He has been smittened with an Australian lady called Una Drake. She has made a bet with him that she can produce an unbreakable alibi where she would be in two places at the same time.
Tommy and Tuppence need to investigate if Una was eating in a London restaurant or staying at a hotel in Torquay that evening.
Tommy and Tuppence head off to Torquay on the train. Tuppence is upset that they forego lunch and end up arriving at the hotel when the restaurant is shut. They do encounter many people who remember Una, from the stewart on the train to the staff at the hotel.
Once back in London they find out from Albert that Una was also dining in London.
It might want to make you think of the Gemini Factor. However when an acquaintance of Una is found dead, Tommy and Tuppence think that there are other reasons she wanted people to think she was in Torquay.
The solution of a woman being in two places at once was obvious. It was then just seeing how the episode advanced its secondary plot. Fun but it was nothing more than average.