The young Venetian Marco Polo is on his way to the Emperor's court in Peking when he meets the intrepid time-travellers, for the TARDIS has landed on Earth in the year 1289. Marco Polo recognises in the TARDIS a means of winning favour with the Emperor. But in the end the Doctor has no one but himself to blame for the loss of his wondrous travelling machine - which he gambles away to Kublai Khan...

The TARDIS materialises in Mexico during the Aztec civilisation. The Doctor and his companions step outside to discover they are inside a tomb - the tomb, it turns out, of Yetaxa, once High Priest of the Aztecs. Barbara is hailed as Yetaxa's reincarnation by Autloc, High Priest of Knowledge, and Tlotoxl, High Priest of Sacrifice, when they find her in the precincts of the tomb wearing the bracelet of the deceased Priest, now revered as a god. And she takes advantage of her position of unaccustomed power to try and dissuade the Aztecs from practising human sacrifice...

The TARDIS lands in Paris on 19 August 1572. Driven by scientific curiosity, the Doctor leaves Steven to meet and exchange views with the apothecary, Charles Preslin. Before he disappears, he warns Steven to stay out of 'mischief, religion and politics.' But in sixteenth-century Paris it is impossible to remain a mere observer, and Steven soon finds himself involved with a group of Huguenots. The Protestant minority of France is being threatened by the Catholic hierarchy, and danger stalks the Paris streets. As Steven tries to find his way back to the TARDIS he discovers that one of the main persecutors of the Huguenots appears to be - the Doctor.