Jace Tanna is having the best summer holidays of his life! Incredibly, he has been allowed to help out at the Mash-Up Championship - a thrilling series of electric, recycled car races through the wild terrain of Rumblemore Island, where Jace lives. The Mash-Up racers are powered by electricity, sourced from renewable energy generated by wind, waves, sun - and the island's smouldering volcano, Mount Rumblemore. All cars are made using recycled parts and the races are planned by the austere Mr Murcan who maps the routes and adds extra challenges - tricky traps and exciting obstacles. Jace is thrilled just to be sweeping the floor for the teams - but then something even more amazing happens. Team Hippo ask Jace to work for them after their mechanic is taken ill - and Jace gets the chance to prove what a brilliant machine-fixer he is, and to become great friends with Team Hippo's daring young navigator, a girl called Lexie. Can the youngest mechanic and navigator in the history of the competition beat the most famous driver of all, Deesol Dragg? Perhaps, but only if they can first prove that Dragg is not even abiding by the rules ...