Under the relentless sun of the Syrian skies, the eccentric English archaeologist Lord Greville Boynton and his son Leonard are seeking the final resting place of the head of John the Baptist. On the verge of this incredible discovery, they are joined by Lord Boynton's second wife, the dominating, fabulously wealthy American Lady Boynton, shadowed as ever by their children — devastatingly handsome Raymond, put-upon Carol, and their troubled younger sister Jinny. Vacationing in Syria, Hercule Poirot is irresistibly drawn to the excavation, but this is soon overshadowed when Lady Boynton is found murdered under the roasting sun.
Commissioned to find her killer by his old friend Colonel Carbury, Poirot soon realizes there is no shortage of suspects. Besides her much maligned family, the dig has attracted an unusual group of onlookers which includes the witty psychiatrist Dr. Gerard, the laconic Jefferson Cope, the beautiful and resourceful Dr. Sarah King, and the unconventional travel writer Dame Celia Westholme.
As Poirot's little grey cells begin to sing and make sense of the case, the mystery suddenly deepens when Jinny and Sister Agnieszka, a Polish nun on pilgrimage, are attacked in the night and the Boynton family Nanny suffers a crushing breakdown. Each of the suspects has a motive for killing the despised Lady Boynton, but only the Belgian detective knows which of them arranged her appointment with death.