It's school holiday time and the JPIs are staying at their favourite place in Portsmouth, UK. The Pelican Hotel is owned by Stan's gran Muriel Larkin and she's always delighted to see them arrive. This holiday is different. A horrible black depression hovers over the hotel. The children sense it straight away but their parents won't discuss it. Stan won't talk, not even to Harry his best friend and locks himself in his bedroom. The JPIs are warned not to step foot on Muriel's private beach because the sand has turned green and the council think it's poisonous. That meant they couldn't go near Sea Shell Cave either. Earth tremors at night are so ferocious they rock the very foundations of the building. Muriel thinks everything is cursed; the beach, the hotel and Sea Shell Cave. The JPIs don't have the same feeling. After noticing strange goings on in the hotel and on the public beach, they think something or someone is responsible for everything that is happening. They intend to find out who and why. It will lead them into life-threatening danger and will be their toughest case yet.