The young Venetian Marco Polo is on his way to the Emporor's court in Peking when he meets the intrepid time-travellers, for the TARDIS has landed on Earth in the year 1289.

Landing on a distant planet, the First Doctor confidently announces to his companions that the TARDIS has brought them to an age of great advancement, peace and prosperity. The Doctor's calculations seem to be confirmed when the travellers are greeted by Jano and the Elders who take them on a tour of their city - a haven of beauty, harmony and friendship, set in a wilderness inhabited by tribes of savages. But the security of the city is founded on one deadly and appalling secret. Soon the Doctor and his friends discover that it is not only outside the city walls that savages dwell...