At the country house of physicist Sir Claud Amory, a formula with potentially destructive military value has been stolen. Amory gathers his family and guests in a darkened room and offers the thief a chance to return it, but when the lights come on he is dead and the document remains missing.
Poirot and Hastings investigate a household marked by debt, romantic conflict and divided national loyalties. Adapted by Charles Osborne from Christie’s 1930 stage play, the novel preserves the interwar fear that private scientific research could become an instrument of international power, while its enclosed setting allows every gesture during the blackout to acquire evidential importance.