Duncan Doncaster

Duncan Doncaster is a non-canonical character created for The Alphabet Murders (1965), the MGM film adaptation of The A.B.C. Murders. He is a psychiatrist who seems to have a link with many of the characters in the plot. He treated Betty Barnard, the second murder victim, much to the disgust of her boyfriend Don Fortune, who suspects, correctly, that he is also interested in her romantically. He also has an affair with Lady Diane Clarke, the wife of the third victim. When Poirot interviews him about a mysterious blonde woman with the initials A.B.C. who seems to be linked with the murders, Doncaster tells him she is yet another of his patients, one Amanda Beatrice Cross. She had schizophrenia and an obsession with the alphabetic sequence. Poirot goes further to ask if Amanda could be manipulated into doing the killings, thus setting Doncaster up for the viewer as the plausible mastermind behind the crimes. Later, Poirot believes Doncaster to harbouring A.B.C. He and Hastings lure him out with a trick "anonmymous" telephone call from Hastings claiming that Poirot is closing in on Amanda. They follow Doncaster to a ship but discover later that Doncaster is the fourth victim.

Doncaster's surname is a nod to the town of Doncaster where, in the original novel, the fourth murder was supposed to take place. In this adaptation, where all the action takes place in London, Doncaster was the fourth person to die. The part of Duncan Doncaster is played by Guy Rolfe.