The Junior Private Investigators are on holiday in Portsmouth, UK. The Pelican Hotel is owned by Stan's gran Muriel Larkin and she's great fun to be with. But not this time, this holiday is different. The JPIs sense it straight away but their parents won't discuss it. Stan is acting strangely; he refuses to talk and locks himself in his bedroom. Muriel's private beach is now out of bounds too. A mysterious green pigment has seeped into the sand, changing its colour. The council were closing it pending laboratory testing which they believed would show the beach to be poisonous. That meant they couldn't explore Sea Shell Cave either. Earth tremors at night are so ferocious they rock the very foundations of the building. Muriel is distraught and thinks everything is cursed, the beach, the hotel and Sea Shell Cave. After noticing strange goings on in the hotel and on the public beach, the JPIs have a different idea on the subject. They believe something or someone is responsible and they intend to find out why. It will lead them into life-threatening danger and will be their toughest case yet.