Famous Five (Book 5) - Five Go Off In A Caravan

In this part all five (Julian, Dick, George, Anne and Timmy) meet a new friend called Nobby and his chimpanzee "Pongo". There's a secret under the ground where the Five have their caravans and the secret is of Nobby's uncle Tiger Dan and his friend Lou. They have had troubles with Tiger Dan and Lou. The mystery starts in one day when the children and Timmy were commanded to camp somewhere else with their caravans. But sooner or later, the two "evil characters" - Tiger Dan and Lou - show up and try to tell them to move. what is going on??...