Two schoolteachers investigate the personal life of one of their brilliant students and her mysterious grandfather.

The TARDIS takes its new passengers back to the dawn of history. There, the Doctor and his party are captured by a tribe who are trying to rediscover the secret of fire.

Old Mother frees the Doctor and the others on condition they will leave the tribe and not make fire.The group attempt to return to the TARDIS but Za and Hur follow them.

Kal accuses Za of Old Mother's murder but the Doctor exposes him as the real killer.However, Za then insists the travellers be held prisoner until they show him how to make fire.

The Doctor, Susan and Ian search the city for Barbara and learn that the area is contaminated with radiation.Then they encounter the Daleks.

Susan meets Alydon of the Thals and agrees to try and arrange a treaty between his people and the Daleks. But the Daleks only see it as an opportunity to wipe the Thals out.

The Doctor and his friends attempt to escape from the Dalek City. However, knowing that the Thals are walking into a trap, Ian decides to go and warn them.

The Doctor and his friends realise that their only chance of recovering the fluid link is to persuade the Thals to fight back against the Daleks.

While Ian, Barbara and their Thal friends try to find a way into the Dalek City through the mountains, the Doctor, Susan and Alydon attempt to make their own way in.

Ian, Barbara and the surviving members of their party try to find a way into the Dalek City whilst Alydon prepares the other Thals for an attack. But only the Doctor and Susan are aware of the full extent of the threat the Daleks pose.

The Doctor becomes increasingly suspicious of Ian and Barbara but when the occurrences aboard the TARDIS become even stranger he realises they are in even greater danger than they realised.

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 travelers 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 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.

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 travelers' only hope for escape lies with Autloc and Cameca.

The ship has been boarded by the Sensorites and the race is on to retrieve Susan and Barbara before they can find them.

The Doctor, Susan and Ian agree to accompany the Sensorites to their home planet in the hope of finding a cure for John. But not everyone is willing to welcome them there.

With Ian having fallen victim to the Sensorites' plague, the Doctor is desperate to find a cure. But the City Administrator is still determined to rid his world of aliens.

The City Administrator is determined to destroy the Doctor and his friends and his paranoia prevents the Doctor's cure from being distributed.

While the City Administrator's schemes reach their climax, the Doctor and Ian venture into the tunnels under the city to discover the real cause of the Sensorite plague.

Susan, Ian and Barbara are sentenced to be guillotined but an encounter with a dying English prisoner may offer Ian a chance of survival.

Susan and Barbara are rescued by Jules and Jean, two members of the escape chain, while the Doctor gains access to the prison by posing as a revolution official.

The Doctor is forced into a meeting with Robespierre while Barbara's attempts to find a physician for Susan result in them being betrayed.

Ian finds himself held prisoner by the treacherous Leon while the Doctor attempts to engineer Susan and Barbara's escape.

Ian finally delivers his message to James Stirling but in order to secure Susan's release he and Barbara are forced into a dangerous spying mission.