The TARDIS lands on the planet Marinus,an odd place with beaches of glass and seas of corrosive acid. Soon they encounter Arbitan,a man who explains to them that the planet was once an enlightened society aided with the help of the Conscience,a computer device that provided much good to the people of the world. That is until the malevolent and power seeking Voord race attempted to gain control of the device. Arbitan anticipated this by scattering the five keys to this device all over the planet and denies the doctor and his companions use of the TARDIS via a force field until they help him locate the keys. All this also because his daughter has gone missing on the same voyage.
The TARDIS crew are all given preset watch like transport devices to take them to the different locations. First the find Arbitans daughter Sabetha and another man named Altos under control of the brains of the city of Morphoton,where the squalor of the city is guised in the illusion of a paradise. Escaping that they arrive in an ice world where a local trapper attempts to lock Altos and Ian away and steal their transport devices for profit. After the murderous trapper is stopped they meet up with the Doctor in their final location where Ian is framed for theft of the last key and it's up to the doctor to prove it was all a conspiracy by the defendant's mentally unstable wife and accomplice. Upon returning with the keys they find Aribitan has been killed and a Voord is now impersonating him. Luckily for the doctor Ian gives the Voord the false key Barbara was lured into Morphoton with and the Conscience is destroyed before the Voord can activate it. Facing both triumph with the loss of their conscience device the Doctor leaves Sabetha and Altos with a warning about self motivation before leaving to continue his journeys.
Of course over a decade later a similar,though longer 'Key To Time' series of serials in the Tom Baker era were aired. It showed again how an idea sprung from an earlier one. As this serial stands it's very much 'The Doctor's Odyssey' as it were. He and his companion are sent from one area of the planet Marinus to another looking for these keys. More importantly we see what an indolent and decadent society Marinus has become,seemingly no thanks to their Conscience device. One city's population is enslaved disemodied brains and has fallen into disrepair,another has a totalitarian legal system where one is "guilty until proven innocent" and,fact is,most natives searching for the key are economically motivated. Seems to be a sutble message here how often people won't effect change until their wallets are tapped out. The ending theme is basically once's dependence on themselves. In the end it's a relatively long (at six parts) but rewarding serial.