In the last episode of the new series, Guy Martin attempts to build and race the world's fastest soapbox racer, creating a unique design, with help from expert designers and bobsled racers, and training with the Team GB women's bobsled pair.

The motorcycle racer leaves his comfort zone as he attempts to set a world speed record for a hovercraft. These machines are notoriously difficult to control, so to learn the basic skills needed he's put through his paces by Royal Marines in a military training operation. To achieve his goal, Guy will need to stay over 86mph for a kilometre, but at these speeds, because of the hovercraft's inherent instability, even the most experienced pilots can lose control - and the worsening weather makes things that much more challenging

Guy visits the USA, where he hopes to win at one of America's oldest races: the Pike's Peak International Hill Climb. In this annual contest, known as The Race to the Clouds, riders compete against the clock on a gruelling mountainside track that finishes over 4000 metres above sea level. To learn the unique skills required in hill climb racing, Guy heads to the birthplace of the sport: Shelsley Walsh hill climb in Worcestershire. Racing up the course in an F1-style car, he realises that sheer horsepower and driving precision are vital. However, in the thin air of Pike's Peak, Guy's body and bike will be desperately short of oxygen, which could seriously affect his performance. As well as trying to add more power to his homemade bike, Guy takes a high-altitude fitness test in a hypoxic chamber. He soon realises how much harder his body will have to work and how the high altitude will affect his brain and cognitive performance; crucial when he has to remember the 150-plus bends in the road and avoid driving over a cliff! After six months' preparation and a two-day road trip across America, Guy's success depends not just on how well he's tuned his bike and body, but on his own abilities as a racer

Guy gets back on a bike to see how far it's possible to cycle during 24 hours of non-stop pedalling. With the help of bicycle brainiacs Miles Kingsbury and Mike Burrows, the man who designed Chris Boardman's gold medal-winning Lotus bike from the 1992 Barcelona Olympics, he builds a revolutionary tandem and pairs up with his friend, endurance expert Jason Miles, to see how far they can push their bodies at the Extreme Environments Laboratory at the University of Portsmouth. In the build-up to the final record attempt at the historic Goodwood Motor Circuit, Guy also learns about the science of cooking for athletes with the Head of Nutrition at Team Sky, Nigel Mitchell, who instructs Guy to lose five kilos of weight and recommends a new diet regime. Guy also works out the best ways of relieving himself on the move to save vital seconds. As the day approaches for the record attempt, news reaches the team that Hurricane Bertha is approaching.