The TARDIS brings the Doctor, Susan, Barbara, and Ian to the planet Skaro where they meet two indigenous races — the Daleks, malicious mutant creatures encased in armoured travel machines, and the Thals, beautiful humanoids with pacifist principles. They convince the Thals of the need to fight for their own survival.
The TARDIS crew lands in the Himalayas of Cathay in 1289, their ship badly damaged, and are picked up by Marco Polo's caravan on its way along the fabled Silk Road to see the Emperor Kublai Khan. The story concerns the Doctor and his companions' attempts to thwart the machinations of Tegana, 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 TARDIS arrives in a desolate future London, where the Doctor discovers his old enemies the Daleks have taken control of Earth.
