The Keeper of Traken is the sixth serial of the 18th season of the British science fiction television series Doctor Who, which was first broadcast in four weekly parts on BBC1 from 31 January to 21 February 1981. In the serial, the alien time traveller the Master (Geoffrey Beevers) seeks a power on the planet Traken known as the Source which he wishes to use to restore his life. The end of the serial introduces Anthony Ainley as the Master. The serial is also the debut of Sarah Sutton as companion-to-be Nyssa. Plot In the TARDIS, the Fourth Doctor and Adric arrive back in N-Space following the events of Warriors' Gate in an area known as the Traken Union, an empire of peace and harmony. They are surprised to find a holographic image of the elderly Keeper of Traken appear in the TARDIS, calling on the Doctor's help. The Keeper explains that his title is about to pass on soon to Consul Tremas, giving him access to the powerful Source that is the centre of Traken's technological advancement, but senses evil within him, his wife Kassia, and his daughter Nyssa. The Keeper suspects a connection to Melkur, an evil creature that arrived years ago on Traken but became calcified in a grove in the capital. Melkur has since become something of a holy symbol, and Kassia has been tasked with talking to it and keeping it clean; that task is soon to be passed on to Nyssa. When the Doctor and Adric land at Traken's capital and visit the Keeper, their presence appears to cause the Keeper to warn the assembled group of a great evil, and though Tremas vouches for them, others, including the Fosters, guardians of the spiritual welfare of the capital, remain cautious about their presence. Soon, bodies in the grove are found, the Doctor and Adric determining they have been killed by some type of plasma weapon. Adric works with Nyssa to identify the energy signature of the plasma as being from a TARDIS, while the Doctor assists Tremas in defusing the conflict over their presence. Unbeknownst to either group, Kassia secretly visits Melkur, who gives her a collar to wear, providing the creature with mind-control over her while promising to keep her husband safe. Kassia is able to convince the Fosters to arrest Tremas, the Doctor, Adric and Nyssa, and uses the situation to convince