Uncle Quontin is in a state. Aunt Frances has to go and stay with a sick relative just as Joan, the housekeeper, has gone on holiday. So Mrs Stick is hired to help look after The Famous Five. The children take an instant dialike to Mrs Stick - and to her horrible son, Edgar, When Uncle Quentin has to go away unexpectedly and Mr Stick appears at the cottage, the children decide to escape to Kirrin Island for a few days. On the island the dungeon entrance is blocked by a huge boulder so the children make an alternative camp in a cave. But they can hear voices. The Sticks are on the island - and Mr Stick is talking about a General and the Gemini Project! The hapless Edgar falls down a hole into the children's cave and reluctantly reveals that Mr Stick has kidnapped the daughter of an army general. They force their way into the dungeons and find Jennifer tied up inside. The Famous Five are freeing Jennifer when they hear Mr and Mrs Stick approaching. The Sticks find a small figure bound and gagged - but it's not Jennifer! The children have tied up Edgar and all three Sticks am now trapped in the dungeons while the children rush to alert the police. The General has been working on a highly secret project which is now safe - and so is Jennifer. The Prime Minister sends thanks to The Famous Five.