1933. Hercule Poirot, older and greyer, receives letters threatening murder. The sender signs themselves only as “A.B.C.” When he takes the letters to the police looking for help Hercule finds all his old friends have moved on. But soon there is a murder, and in order to have any hope of catching the killer, the once great detective must take matters into his own hands.

Hercule is held in deep suspicion by Scotland Yard and the A.B.C. killer sends more letters promising to spill blood. Against a background of xenophobia in Britain, Hercule’s past, integrity, and identity come into question. But a friend from Hercule’s past intervenes and employs him to put an end to the slaughter.

With the A.B.C. killer still on the loose, Hercule realises that only he holds the key to the murders. He faces a race against time to stop the bloodbath.