Inside The Spaceship Following an explosion, unexplained events occur inside the TARDIS. The Doctor and his companions turn on one another as they attempt to discover who - or what - is at fault... Marco Polo The TARDIS has arrived on the Plain of Pamir in 1289. The crew join Marco Polo and his caravan on a long journey to Peking to meet the mighty Kublai Khan. The Keys of Marinus The travellers are compelled to find the keys to the Conscience of Marinus, she decides to change history for the better. The Aztecs When Barbara is heralded as the reincarnation of the high priest Yetaka, she decides to change history for the better.