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, in search of the lost Incan city of Marpa. The search takes them ever deeper into the canyon. As the team races to reach camp by nightfall, nurse Mandy slips and injures her ankle on the trail. She is confronted with the agonising prospect of being sent home.