Jem faces his toughest challenge yet: to build a boat made out of ice, and take it and the rest of the team for a spin in one of the UK's busiest stretches of water. Dallas investigates the world of comets and tries to build his own, while Liz looks at the science behind hypothermia and gets repeatedly dunked into ice-cold water in preparation for the ice boat extravaganza.

• Dallas Campbell
• Liz Bonnin
• Jem Stansfield