The motorbike fan takes on the challenge of building the world's fastest soapbox racer in an attempt to set a new speed record using just the pull of gravity and a tarmac road. To get to grips with going blisteringly fast on a public road, Guy tries his hand at street luge with former global champion Helene Schmit, before joining his team of engineers to explore the science of rolling resistance and the importance of tyre pressure. Armed with what they believe to be the perfect formula, they produce a unique soapbox cart inspired by Mother Nature and, after training with the Team GB women's bobsled pair, confidence is high - but are disaster and mayhem just around the corner?