The Doctor with Susan and Ian

The TARDIS's next landing point is a snowy landscape when The Doctor reports that the TARDIS has developed a major fault: all the lights have failed, the water supply has been affected and the heating is inoperative. Which means they are now in danger of freezing to death.

Exploring the snowy plateau, The Doctor, Susan, Barbara Wright and Ian Chesterton are confronted by a group of Mongol warriors led by a man named Tegana who prepares to have them put to death as evil spirits. They are saved by the arrival of a man who Barbara Wright later identifies as the Venetian explorer Marco Polo.

The year is 1289 and Marco Polo is taking a caravan along the fabled Silk Road from the snowy heights of the Pamir Plateau, across the treacherous Gobi Desert and through the heart of imperial Cathay en route for the court of Emperor Kublai Khan.

Transportation

Marco Polo seizes the TARDIS, intending to present it as a gift to the Emperor in the hope that he may then be allowed to return to his native Venice, and The Doctor's group have little choice but to join the caravan. Unbeknown to them all Tegana also secretly plans to steal the TARDIS, but for his own master Noghai.

Mistrust and acrimony abound as the caravan makes its way through the Pamir Plateau, across the treacherous Gobi Desert and through to Cathay, stopping every so often at way stations along the route.

Marco Polo, The Doctor, Susan and Ian

It is left to The Doctor and his companions to thwart the machinations of Tegana and his Mongol allies who make a number of unsuccessful attempts to sabotage the caravan and to kill the rest of the party. But the time travellers are unable to convince Marco Polo of the man's treachery or to regain access to the TARDIS, which The Doctor has in the meantime managed to repair.

The caravan finally arrives at the Khan's summer palace in Shang-Tu, where the travellers have an audience with the ageing leader who is at the height of his imperial power. The Doctor and Kublai Khan strike up a cordial relationship, which continues when the whole party moves on to the Imperial Palace in Peking. This leads to a game of backgammon between the two old men, with the TARDIS as the prize. Unfortunately, The Doctor loses.

There is some good news, however, as Ping-Cho learns that her intended husband, who she does not love, has died after drinking an aphrodisiac, allowing her to continue her romance with dispatch rider Ling-Tau.

Barbara and Ian

Tegana now reveals his true intentions - to assassinate Kublai Khan and so leaving the way clear for Noghai’s army to attack and overrun Cathay. However, Marco Polo, having been alerted by the time travellers and Ling-Tau, saves the Khan's life by engaging Tegana in a sword fight and overpowering him. Humiliated, the Mongol takes his own life by throwing himself upon a sword.

In defeating Tegana, Marco Polo’s is awarded with regaining the Emperor's respect. Therefore Marco Polo returns the TARDIS key to the travellers in gratitude for what they have done, so allowing them at last to depart. The Khan is philosophical, commenting that The Doctor would have won it back at backgammon in any case…