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, afraid that schoolteachers Ian Chesterton and Barbara Wright will reveal his secrets to the people of 20th century Earth, has taken 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!

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 28th century. The TARDIS materialises on a spaceship orbiting a planet known as the Sense-Sphere, whose natives, the Sensorites, have trapped the ship's human crew in a state of paralysis.

The Doctor and his companions are on board an Earth ship trapped in orbit around the Sense-Sphere when it is boarded by the Sensorites, who steal the lock mechanism from the TARDIS.

The Doctor and his companions are escorted to the Sense-Sphere where they attempt to negotiate with the Sensorites for the release of the TARDIS.

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.