In the novel The A.B.C. Murders, Capper is a serial child murderer who was caught by Inspector Crome much to his credit. One of Capper's victims, his third, was a schoolgirl named Mabel Homer. Crome told Poirot that Capper was extremely difficult to pin the crime on but Crome managed to to do it through some modern psychological techniques involving tests which exploited verbal traps. According to Crome, once a man is induced to give himself away, his nerve goes and he starts revealing everything. At the time of the events in the book, Capper is in Broadmoor.