The TARDIS experiences a total power failure and is stranded in the Himalayas in the 13th century. The travellers are forced to join the caravan of the famous explorer, Marco Polo, in order to survive -- but Polo, fascinated by their claims that the TARDIS can fly through the air, seizes it on behalf of Kublai Khan in the hope that presenting the Khan with such a fabulous gift will convince him to let Polo return home to Venice. The travellers are forced to accompany Polo on his long trek across Asia to the Khan's court, but their many attempts to reclaim their property are foiled -- not just by Polo, but by the war lord Tegana, who is apparently on a mission of peace but has entirely different plans for dealing with Kublai Khan...

Tegana sabotages the party's water bags, leaving them in dire need of fresh supplies.

The Doctor and Susan are able to supply Marco with condensation that has formed inside the TARDIS, saving the group from dying of thirst. When the group are reunited with Tegana, the travellers begin to suspect he is up to something.

The Doctor's group manage to rescue Barbara from the Mongols but once more Marco remains unaware of Tegana's involvement.

The Doctor and his friends elect to warn Marco of Acomat's attack even though it means losing their chance to gain access to the TARDIS.

The Doctor, Ian and Barbara are forced to surrender to Tegana and Marco, leaving both of them the opportunity to gain possession of the TARDIS.

Tegana finally makes his move to assassinate the Khan, giving the Doctor and his friends an opportunity to thwart his plans and regain the TARDIS.

The TARDIS arrives on Earth landing in 16th century Aztec society. Barbara goes off exploring - despite the Doctor's orders not to wander off - and is taken away by the high priest Autloc who believes she is the reincarnation of the goddess Yetaxa. They believe it is a sign that the rains - which are late - will now come. The high priest of sacrifice Tlotoxl recruits Ian to lead their army but he finds he has a rival in Ixta. Ian is less than comfortable with his role when he learns that they are to deliver the human sacrifice to the high priest at the rain ceremony. The Doctor emphasizes that they must not interfere with Aztec rituals, something that does not sit well with Barbara.

Barbara tries to change the course of history - and Ian fights for his life.

While the Doctor tries to find a way back to the TARDIS, Tlotoxl continues to plot Barbara's downfall and sees an opening when Susan questions Aztec custom.

Tlotoxl's machinations result in both Susan and Ian being arrested. With the TARDIS once again trapped inside the tomb, the travellers' only hope for escape lies with Autloc and Cameca.

The TARDIS lands in sixteenth century Paris and Steven becomes involved with a group of Huguenots.

While trying to find the Doctor, Steven discovers a conspiracy to assassinate a man known as the Sea Beggar.

Steven has discovered the identity of the Sea Beggar, but can he warn the Huguenots in time?