The Keys of Marinus

The Doctor, his granddaughter Susan Foreman, and her teachers Ian Chesterton and Barbara Wright arrive on a small island on the planet Marinus where they meet Arbitan, Keeper of the Conscience of Marinus—a vast computer developed as a vast justice machine which kept law and order across the entire planet.

In Morphoton, the crew are impressed by the luxuries of the city; however, Barbara soon realises that something is not right.

While the Doctor continues to the City of Mellennius, the others search in a dangerous screaming jungle.

Ian and Barbara retrieve a key and teleport to an icy wasteland. They meet the duplicitous trapper Vasor, who steals their keys.

At the next location, Ian is accused of the murder of Eprin, a friend of Altos. At Ian's trial, the Doctor returns and postpones the trial while he gathers evidence.

The travellers return to Arbitan's island, where Altos and Sabetha have been held prisoner by Yartek—Arbitan's killer—and the four keys have been seized.