On holiday in Jerusalem, Poirot hears about an archaeological expedition to Syria led by the eccentric Lord Boynton and his son Leonard, who believe they are on the track of the head of St John the Baptist. Drawn to visit the dig, Poirot meets Boynton's dominating, fabulously rich, American-born second wife. However, events are overtaken by the discovery of Lady Boynton's dead body and revelations about her fortune.
Poirot is given the task of finding the killer. As ever, he has no shortage of suspects.