The Lernean Hydra
One of Poirot's twelve labours - the Lernean Hydra appears in the form of ill-intentioned gossip, aimed at a physician accused of poisoning his wife. From The Labours of Hercules.
More about this story
The hydra is a many-headed creature, in this case for Poirot it manifests itself as the many-headed monster of gossip and rumour which is ruining the life of Dr Charles Oldfield, rumoured to have poisoned his wife.
The story was first published in 1939 in Strand Magazine (UK) and in This Week (US), under the revised title Invisible Enemy. It forms one of The Labours of Hercules. The collection The Labours of Hercules was adapted in 2013 as part of the final series of Agatha Christie's Poirot, starring David Suchet.