Directed by Mike Marzuk [de], it is a film adaptation of The Famous Five series by Enid Blyton, which is based primarily on the 1947 book Five on Kirrin Island Again.

Georgina "George" Kirrin, Quentin's daughter, is forced to go cycling with her cousins and decides to take them to the "Teufelsfelsen" (Devil's Rock).

She leaves and cycles to the harbour to say goodbye to Quentin, who then travels to "Felseninsel" (Kirrin Island).

At Teufelsfelsen, the siblings hear a dog barking and, ignoring a sign that says "stay out, dangerous," enter a cave.

Timmy is banned from George's house, as he accidentally destroyed parts of Quentin's work and now lives with neighbour Mrs. Miller.

Examining a piece of paper Dick picked up in the cave earlier, George finds the imprint of a signature.

While collecting signatures for a fake ecological campaign, Anne learns that a couple of birdwatching aficionados wants to visit Felseninsel.

After failing to have found matching signatures, the friends investigate the couple, whose hovercraft can reach Felseninsel.

Later, George reads the fine they got for illegally entering the cave and realizes that Hansen's signature is the one the friends were looking for.

She is locked up with Turner in a cell under the lab, while Timmy flees and chases the bus carrying the siblings home.

The criminals are brought to prison by Turner, who promises that his agency will compensate the bird watchers for their troubles.