Wouldn't you just know it, there's a murder on the plane on which Poirot is traveling from Paris to London. One Madame Gisele is killed via poisoned dart. Madame Gisele was a moneylender to the upper crust and also a blackmailer. Poirot has seen her with the haughty Lady Horbury, but Lady Horbury claims never to have seen Madame Gisele before. Poirot is troubled. A wasp is found on the plane, killed by one of the passengers with his coffee cup. Poirot wonders what the wasp was doing there. And how did someone kill Madame Gisele with a dart in front of all the passengers? It's a knotty problem, which Poirot finally solves, but it's much more complicated than it appeared in the beginning with situations involving bigamy and fake identities.
Excellent mystery with Poirot and Japp working together, and Poirot enlisting the aide of the female flight steward as well. The usual high production values abound.