S03E04 - Wasp's Nest

Sat on the garden terrace of his large house on a summer evening, John Harrison is delighted to receive an unexpected visit from Hercule Poirot. The detective tells him that he is in the locality to investigate a murder but amazes Harrison by admitting that the murder has not yet been committed but that it is better to prevent one happening first. He then cryptically turns the conversation round to a wasp's nest on a tree that he sees nearby and Poirot and Harrison discuss the destruction of it. A friend of Harrison's called Claude Langton is going to handle the task for him using petrol injected by a garden syringe but Poirot tells him that earlier that day he saw the poison book in a local chemist's and an earlier entry made by Langton for the purchase of cyanide, despite Langton having told Harrison that such substances shouldn't ever be used. Poirot asks a quiet question: "Do you like Langton?" and they talk of Harrison's engagement to a girl called Molly Deane who was previously engaged to Langton. As they discuss Langton's disposition towards Harrison, Poirot's meaning becomes quite clear to him – he is being warned. Poirot asks when Langton is returning and is told nine o'clock that evening. Poirot tells him that he will return at that time, wondering as he goes if he should have waited behind.