After discussing a strange pupil, Susan Foreman, teachers Ian and Barbara follow her to a junkyard. They meet her grandfather, the Doctor, and force their way inside a police box with frightening results.

The enigmatic Doctor, wary that Ian and Barbara will reveal his secrets to the people of 20th-century Earth, takes them and his granddaughter Susan back to the dawn of human history. There, the four travellers are dragged into the savage politics of a tribe of cavemen who have lost the secret of making fire...

Prisoners of the earliest humans, the TARDIS crew are caught in a deadly power play for leadership of the tribe.

The TARDIS crew must make fire for the early humans to stay alive. But once they do, they may not be allowed to leave!

Time Unknown, Planet Unknown. The TARDIS lands in an eerie petrified forest overlooking a magnificent steel city. The Doctor wants to explore the city – but what's waiting there?

The dead planet is not uninhabited after all. The Doctor is about to meet what will become his greatest enemies... And he and his crew are dying of radiation sickness.

Imprisoned on Skaro by the scheming Daleks, the Doctor and his companions work to escape their clutches.

The time travellers work desperately to both escape the Dalek city and prevent a bloody massacre.

The time travellers have escaped from the Daleks, but left a vital piece of the TARDIS behind. Getting it back will mean pushing the Thals to mount a dangerous operation.

Ian, Barbara and the Thals struggle through the caves to reach the Dalek city. Will they get there in time? Is there even a way through the treacherous caves? And what are the Daleks planning?

Time is running out as the Doctor and his companions try to defeat the Daleks before they wipe out all life on Skaro.

A mysterious explosion has knocked the TARDIS into void. And in their confusion, the crew are turning on one another.

The TARDIS is adrift, the crew turning on each other. Only by working together can they find their way out before they are destroyed.

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 alien world of Marinus. Perfect peace has reigned here for decades due to the mind-controlling influence of a machine known as the Conscience. But now a race known as the Voord have developed an immunity and Arbitan, keeper of the machine, has been forced to scatter the vital control keys around the planet. Now he needs the help of the time travellers to get them back - it will be an extraordinary journey, with perils every step of the way...

The Doctor and his friends arrive in the apparently idyllic city of Morphoton but Barbara soon begins to realise there is more to their hosts than meets the eye.

The search for the second key takes the travellers to a jungle where Ian and Barbara come under threat from rapidly growing plants.

Searching for the third key, Ian and Barbara meet with the sinister trapper Vasor before following Susan and the others into a cave system guarded by the mysterious Ice Soldiers.

The group are reunited with the Doctor in the city of Millennius where Ian faces trial for murder after the fourth key is stolen.

The Doctor and his friends must locate the real murderers and the fourth key before heading off for a final confrontation with the Voord

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.