Ben Fogle is on an epic South American adventure across the Andes from the Equator to the Tropic of Capricorn. He is now in southern Peru on the third leg of his journey, leading a new team of heroes into the remote Cotahuasi Canyon, the deepest gorge on the planet. Ben and his team make the final push to their ultimate goal - the lost Incan city of Marpa. They risk life and limb on a dizzying 2,500m descent straight down the sheer canyon wall. With terrifying drops either side, the team are forced to push their bodies to their physical limits. When charity worker Ann dislodges a rock, it crashes into team-mate Giles below, threatening to end his dream of making it to Marpa.