Serial: E  Episode Nos. 21, 22, 23, 24, 25, and 26.  Title: The Keys of Marinus

Landing on an island in a sea of acid, the TARDIS crew are forced on a quest for the lost Keys to the Conscience Machine of Marinus, a device that will enforce justice in the minds of everyone on the planet.  The four missing keys are scattered across the planet, and using teleportation "travel dials," the Doctor and his companions must follow the trails across varied locations and terrains and return them to the island before the villainous Voord take control of the Conscience Machine...

starring William Hartnell as Dr. Who, William Russell as Ian Chesterton, Jacqueline Hill as Barbara Wright, and Carole Ann Ford as Susan Foreman.  Written by Terry Nation, Associate Produced by Mervyn Pinfield, Produced by Verity Lambert, and Directed by John Gorrie.

Originally transmitted from 11 April 1964 to 16 May 1964 on BBC1.