Travelling to Bath, England, Dick and Jem meet up with an eco-minded sandwich shop owner eager to create an environmentally-friendly delivery vehicle. Riding around town on a tandem bike, Dick and Jem experience first-hand the challenges of creating a green vehicle able to cope with the hills and cobblestones of Bath. To build a lightweight ‘green machine’, Dick and Jem will need to make something small and agile – able to accelerate uphill and manoeuvre through traffic. The solution: an air-propelled motorbike. To test the idea, Jem catapults himself and his bike off an air-powered ‘cannon’ using a single piston engine. But, for the bike to actually work, the duo must squeeze 5,000 litres of air – an amount that would take the space of an elephant at normal pressure – into a tank that will fit on the bike. This much compressed air could cause an explosion if involved in a collision so Dick and Jem must develop proper safety measures. The biggest test will be getting the air-propelled motorbike to pass government inspection.