The story gets under way at a weekend house party where a scientist is murdered and his secret papers stolen. Putting his "little grey cells" in action, Belgian detective Hercule Poirot methodically pieces together the clues, revealing the culprit to be -- you guessed it -- the Least Likely Suspect.

The naval officer Maurice de Vergnes, tired of the extravagance of his fiancée Moussia, finds with pleasure a childhood friend: Anna. Jealous, Moussia breaks up with him and Maurice marries Anna. A few years later, Moussia reappears when war is declared. She is now a spy under the orders of Howell... This is a troncated version of a longer film.