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. Plot 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. Production 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. Holmes said that he considered the idea of a large monster a mistake given the budgetary constraints at the BBC and named The Power of Kroll as his least favourite Doctor Who story. Extensive location filming took