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Arbitan guarded the Conscience of Marinus and was unable to ever leave the island on which the supercomputer was housed. After the rise of Yartek and the Voords, the five key microcircuits were removed and hidden to keep the Conscience from them, with Arbitan retaining one and being the only one to know where the other four were placed.
On the island, Arbitan worked on the Conscience so that it would be irresistible and thus capable of taming the Voord, who made repeated attempts to claim the machine. Arbitan sent his followers, including Eprin and Altos, after the keys but they never returned. Some years after the Voords' last attack, he sent his daughter, Sabetha, who also failed to return.
Renewed hope and death
A year after Sabetha's departure, Arbitan saw the TARDIS materialise on the island. He watched the First Doctor, Susan Foreman, Ian Chesterton and Barbara Wright and treated them as potential enemies until Ian saved him from a Voord, after which he was filled with renewed hope. He also saved Susan by stabbing and killing a Voord.
He told them about the keys, but they refused, prompting him to place a force field around their ship until they agreed. Arbitan gave the strangers travel dials and asked them to attempt to gain news of Sabetha. Shortly after they left, Arbitan was stabbed and killed by a Voord who had been hiding nearby. (TV: The Keys of Marinus)