While Hercule Poirot alerts Scotland Yard about receiving mysterious letters from A.B.C., Cust intends to start his business, as a travelling salesman. After receiving a forewarning letter from A.B.C., Alice Asher in Andover is found dead with an A.B.C. Railway Guide beside her. Soon after, the threat of the second murder arrives before Betty Barnard in Bexhill, is found dead in a similar fashion. Unknown to Hercule, a few hours before her death, Betty had sampled a pair of Cust’s stockings.
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 rising 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.
At last, the police put their faith in Hercule’s theories. But the A.B.C. killer is still on the loose and is promising to kill again. Hercule realises that only he holds the key to the murders. But the pressure is mounting on Inspector Crome and the public demands answers.
Hercule faces a race against time to stop the bloodbath. Just as the police believe they have caught their man, Hercule suspects further artifice. Can he fulfil his life-long debt to the dead, solve the mystery and learn the true identity of the A.B.C killer?