After collapsing in front of his 'rival' Japp, being rushed into hospital and diagnosed with a weight problem that could cause a heart problem, private society detective Hercule Poirot, who must recover and hopes to lose some weight, visits a seaside health farm on "Burgh Island", in Devon, which has about the best weather in Britain, accompanied by his British friend and assistant, major Hastings. For once he needs to keep far from mysteries, but a murder happens right there: the flirtatious heiress Arlena Stewart is found strangled on a secluded beach where she went sunbathing near a cove. As usual Poirot has screens several suspects such as Christine Redfern, whose husband, journalist Patrick Redfern, was flirting with Arlena in front of Christine while Arlena's husband Kenneth Marshall and 17-year-old step son Lionel Marshall resented her own flirtations with other men. Others had much to loose by Arlena's diva-bitching in professional life. A strange guest at the resort, Major Barry, rather grimly warns Poirot and Hastings to leave as the island isn't a safe place but refuses to explain why. The mystery unravels slowly as he struggles with the exact method of the killer(s?) and especially the timing of events, for both deliberate deception and chance complicated matters.