Well Done, Secret Seven

The Secret Seven have built a new meeting place -a tree house! - and they can't wait to start using it! But they soon realise that they're not the only ones using the tree house - somebody else is using it, too. When they find out who it is, the Seven are outraged. But then they discover that their intruder is in trouble, he's on the run from a gang of thieves, and he needs their help. But are the gang up to the job?