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

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.