Why Didn't They Ask Evans?

An adaptation of a 1934 mystery novel by Agatha Christie, featuring the one-shot characters Robert "Bobby" Jones and Lady Frances "Frankie" Derwent as the co-protagonists. During a game of golf at the Welsh seaside town of Marchbolt, Bobby hears a man screaming. He decides to see if someone needs help, and he discovers a mortally injured man who apparently fell down a nearby cliff. The dying man asks "Why didn't they ask Evans?" before he dies. Bobby has no idea who the man was, or who was Evans. When an older woman identifies the dead man as her brother, and interrogates Bobby concerning her brother's last words, Bobby is moderately suspicious of her story. When Bobby suddenly receives an unexpected job offer from Argentina, he is puzzled why would anyone be interested in him. When Bobby barely survives a poisoning attempt, he starts believing that someone is trying to get rid of him. He asks for the help of his childhood best friend Frankie, who starts investigating about the dead man's real identity. She soon learns that the man was the well-known world-traveler Alan Carstairs, and that he had been investigating the circumstances behind the death of one of his best friends, the millionaire John Savage. As Bobby and Frankie start investigating both deaths, one of their suspects dies in an apparent suicide. The two amateur detectives try to find out what is going on with the series of sudden deaths, while also trying to survive attacks by mysterious foes.