The Belgian detective Hercule Poirot travels to what appears to be an idyllic vacation island in the Adriatic Sea. The Evil exists anywhere.... even in paradise islands.A former actress (Arlena Marshall), who alienates everyone around her, is found strangled on the beach. Everyone in the resort had a motive to kill Arlena, but all seemingly have an alibi. Poirot will investigate the murder. Poirot discovers that the murderer is Patrick Redfern, Arlena's lover, who had been having an affair with Arlena while seemingly flaunting it in his wife's face. He wanted to steal back a diamond he had given her, which had itself been stolen before Arlena came into possession of it. It turns out that not only did Patrick's wife approve of the affair, but was in cahoots with her husband. An elaborate ruse had been set up to throw Poirot off the track with Patrick's wife temporarily impersonating Arlena after Arlena had been killed. The murder turns out to be linked to a previous murder case, in which it was assumed that a vagrant had committed the crime.