This ambitious six-part serial concerned the search for five keys to restore the Conscience of Marinus, the computer that maintained law and order on the planet Marinus. In the process, The Doctor and his companions travel to two cities, a sentient jungle, and an icy wasteland.
The TARDIS arrives on a small island in a sea of acid, where Arbitan (George Coulouris) the Keeper of the Conscience, tells them of the Voord, a species that wants to take control of the great computer. To prevent this, it is necessary to obtain five keys hidden on the planet. To force The Doctor and his companions to help, Arbitan places a force-field around the Tardis, just before he is assassinated by the Voord.
They teleport to the city of Morphoton, which they believe is a luxurious paradise, but they have been hypnotized and it is in fact squalid and filthy. They find the first key on the neck of a slave-girl and start a revolution to free the hypnotized slaves. The Doctor goes on to the City of Mellennius and the others to a screaming jungle, where the plants are in the habit of strangling people. In an ancient temple they find an aged scientist who reveals the location of the second key before dying. They teleport to an ice-bound wasteland, over-run by wolves, where, after retrieving the keys stolen by a larcenous trapper, they discover the next key in a block of ice. In the final location, Ian is framed for murder. The Doctor rejoins them in time to prove Ian's innocence and prevent his execution. The last key is hidden in the murder weapon.
The absence of The Doctor in the 3rd and 4th episodes was so William Hartnell could take a vacation. He had health problems throughout most of his tenure as the Doctor. The Keys of Marinus did not get a lot of love. The story, though adventurous, was way too ambitious for the set-budget and often came off cheap-looking.