The TARDIS crew lands in the Pamir Mountains of the Himalayas in 1289, their ship badly damaged, and are picked up by Marco Polo's (Mark Eden) caravan on its way along the fabled Silk Road to see the Emperor Kublai Khan (Martin Miller). The story concerns the First Doctor (William Hartnell), his granddaughter Susan Foreman (Carole Ann Ford) and her teachers Ian Chesterton (William Russell) and Barbara Wright (Jacqueline Hill), and their attempts to thwart the machinations of Tegana (Derren Nesbitt), who attempts to sabotage the caravan along its travels through the Pamir Plateau and across the treacherous Gobi Desert, and ultimately to assassinate Kublai Khan in Peking, at the height of his imperial power. The Doctor and his companions also attempt to regain the TARDIS, which Marco Polo has taken to give to Kublai Khan in effort to regain the Emperor's good graces. Susan gets the TARDIS key from Ping-Cho (Zienia Merton]]) but is captured by Tegana before they can depart. They are finally able to thwart Tegana, who kills himself before he can be executed, restoring the Emperor's respect for Marco Polo, and the Emperor allows them to depart.