Meet Cass and her best friends, Lex and Nicholas – AKA the Bubble Street Gang! They investigate crimes, solve mysteries and have brilliant adventures. They've even got their own secret clubhouse. Join them as they investigate two mysterious mysteries ... and go on other crazy adventures! The Clubhouse Mystery: After the annoying baby twins demolish the fort Cass has built in the sitting room, she has the great idea of building a proper fort outdoors that can be the Bubble Street Gang's clubhouse. Lex's granny suggests the perfect spot at the back of Mr McCall's field and Cass and the gang get to work. The gang settle in quickly and begin planning their next adventure, when they realise someone is using the clubhouse when theyre not there. Stuff moves around, cookie crumbs are left behind and the door is left open ... Making Millions: The gang need money and quick and they have some super-genius ideas on how to make it! Like Operation Start a Company, Make Loads of Money for Nicholas's Project and Probably Also Become Millionaires! But Cass also has a mystery to solve. Who is the invisible boy? And why can't anyone else see him?