The Percival diamond disappears when twins, William and Elsie Percival, are children. Not too long afterwards, their parents die. They unearth a trunk in the attic; the contents : the secret to the Percival diamond, and set out to find out what happened to it. A weekend party to unravel the secret results in a poisoning, shooting and much more.

Multi-millionaire Walter Sinnet is crooked and ruthless. He builds an impressive underwater folly on his property. Wishing to show off, Walter welcomes a house party to his home, but an angry storm lashes down on the village and traps everyone inside the estate. A guest’s jewellery goes missing and a dead body is found floating in the lake.