Diamond heiress Olivia Prescott is disturbed by an enigmatic stranger arriving at her villa, claiming to be her supposedly dead brother. He seems to know every trivial detail of their past family life, his papers verify his identity, but Olivia insists with mounting hysteria that he is an imposter despite his convincing everyone around them. Is Olivia going mad ... or is she concealing something? And what about the £10 million worth of diamonds that have gone missing?