Mystery of Three Quarters

Hercule Poirot investigates a baffling mystery; why have four people all received a letter supposedly signed by him, which accuses each one of them of murdering Barnabas Pandy? Poirot has never heard of a Barnabas Pandy, and has accused nobody of murder. Who is Barnabas Pandy, is he dead, and, if so, was he murdered? And can Poirot find out the answers without putting more lives in danger?