Hercule Poirot finds an angry woman waiting outside his front door one day.
She demands to know why Poirot has sent her a letter accusing her of the murder of Barnabas Pandy, a man she has neither heard of nor ever met.
Poirot has also never heard of a Barnabas Pandy, and has accused nobody of murder.
Then he has another visitor waiting for him - a man who also claims also to have received a letter from Poirot, accusing him of the murder of Barnabas Pandy.
Who sent these letters, and why? And who is Barnabas Pandy?
And can Poirot find out the answers without putting more lives in danger?