The Percival diamond disappears when twins, William and Elsie Percival, are children. Not too long afterwards, their parents descend to their deaths over the edge of a cliff, leaving them the family estate coupled with a vast fortune.
William and Elsie Percival grow up living a debauched life of partying, opulence and luxury. Soon enough, their money begins to dwindle, they have to concoct a plan to revive their waning funds. During one of their many parties, they unearth a trunk in the attic; the contents : the secret to the Percival diamond that allegedly went missing decades before. It seems Mary, their cousin, whom the siblings have not seen in many years, may be able to unlock the mystery and so a house party is organised in her honour.
Mary, a prickly tempered spinster, takes the train to Oakmere and in her carriage she encounters a peculiar company, all of whom seem to have an inkling about the coveted jewel. The guests that arrive at Oakmere Hall are a mixed bunch. Death also pays a visit.
Death is an insomniac that weekend: a poisoning, shooting and a fall bring Inspector Thomas and his bumbling sidekick Constable Turner hurrying up to the estate to try to get to the bottom of the enigma.
Finally the mystery is solved, but is this the end, or has a plan for revenge just begun?