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The Power of Kroll is the fifth serial of the 16th season of the British science fiction television series Doctor Who, which was first broadcast in four weekly parts on BBC1 from 23 December 1978 to 13 January 1979. The serial is set on the third moon of the human colony world Delta Magna. In the serial, a methane catalysing refinery's operations in the swamp disturbs the giant squid Kroll, which grew in size after ingesting the fifth segment of the powerful Key to Time. When Kroll surfaces, it subsequently rampages on the moon.

The Fourth Doctor and Romana have arrived on the third moon of Delta Magna, searching for the penultimate segment of the Key to Time. They find themselves caught in the middle of a dispute between the crew of a methane refinery and the natives (known as 'Swampies'). The Swampies claim that the crew have disturbed the waters, and will incur the wrath of their god, Kroll. Kroll is revealed to be a giant squid, which surfaces to feed every few centuries and is the source of the abnormally large amount of methane being mined. Originally a normal size squid, Kroll ingested the fifth segment of the Key to Time and began to grow, becoming a god like figure to the Swampies and their descendants. After Kroll awakens and begins to attack both the Swampies and the refinery indiscriminately, the Doctor uses the tracer to retrieve the segment of the Key, which destroys Kroll and instigates cellular regeneration within the creature to produce hundreds of normal sized squid. This process not only saves the planet's inhabitants but removes the source of methane from the refinery. The Doctor and Romana return to the TARDIS and set off on their next adventure.

When script editor Anthony Read asked Robert Holmes to write the story, there were two requirements: that it include the largest monster in series history and that Holmes minimise the humour that many scripts from the era were known for. This second requirement was a request from higher up at the BBC. Early titles for the story were The Shield of Time, Moon of Death and Horror of the Swamp. The serial was directed by Norman Stewart, who had directed the Underworld story a year previously. This was his final assignment on the programme. The actors playing Swampies were coloured green with a special water resistant dye ordered from Germany. Unfortunately, the make up artist failed to order the special dye remover, with the result that many of the actors had to take chemical baths to get the green dye off, and many had a green tint for a long period after production was finished. Costume designer Colin Lavers introduced a new tweed coat for the Doctor, which sported four flying duck brooches on its lapels. Around this time, producer Graham Williams fell ill and his duties were taken on by Anthony Read and production unit manager John Nathan Turner, assisted by Blake's 7 producer David Maloney.

Among the cast were a number of actors who had previously appeared in Doctor Who playing other characters. The most notable of these was Philip Madoc who had previously appeared in The Krotons (1968–69), The War Games (1969) and The Brain of Morbius (1976). He had initially been invited to play Thawn, but the role was given to George Baker, who then pulled out. Madoc agreed to play the part of Fenner when Alan Browning, who was slated to play Fenner, fell ill before the start of production, but was dissatisfied with the role. John Leeson, best known as the voice of K9, appears in this story as Dugeen. while John Abineri previously played Van Lutyens in Fury from the Deep (1968), General Carrington in The Ambassadors of Death (1970) and Richard Railton in Death to the Daleks (1974).

A novelisation of this serial, written by Terrance Dicks, was published by Target Books in May 1980.

The Power of Kroll was released on VHS on 5 June 1995. This serial, along with the rest of season sixteen, was released as part of the Key to Time box set on region 2 DVD on 24 September 2007, and in Region 1 on 3 March 2009.