The TARDIS, chasing the pieces of the Key to Time, has arrived on the third moon of Delta Magna. The Doctor and Romana find themselves in the middle of a dispute between the crew of a methane refinery and the natives—the workers call them Swampies—of the marshlands. The natives accuse the crew of disturbing the waters and incurring the wrath of their god Kroll. This is a giant squid-like creature which comes to the surface to feed every few centuries and produces the methane the refiners covet. Once it was a normal squid, but after ingesting the fifth segment of the Key to Time, it began to grow, and now it is a god.
The Doctor is shot at by refinery workers. Romana is taken to be sacrificed to Kroll—what else—in a scene entirely too reminiscent of Fay Wray’s sacrifice to Kong. Meanwhile, a gunrunner is providing weapons to the Swampies, ostensibly on behalf of a sympathetic organization, but it’s really part of a plot to destroy the natives. The Doctor rescues Romana, but then she is captured again, and the Doctor, and the Swampies turn on the gunrunner, and all three of them are threatened with sacrifice to Kroll, but the Doctor does a Houdini and they escape.
Kroll awakens and attacks both the Swampies and the refinery. The crew of the latter try to kill Kroll with a rocket, but the Doctor disables it. Instead, he uses his tracer to retrieve the Key segment, which begins a process of cellular regeneration, causing Kroll to break down into hundreds of normal-size squid. This reduces the abnormally large amount of methane in the waters, so the refinery will have to be dismantled—what little Kroll left of it—and the crew will move on, leaving the swamp to the Swampies.
The scriptwriters were instructed to create the largest monster in Doctor Who history. Kroll is two miles wide. The serial was filmed in Suffolk, in the marshes of the River Alde. John Leeson, the voice of K9, played a character named Dugeen because K9 did not appear. A swamp is no place for a robot dog. I am reminded of the few human roles that Andy Serkis has played. Aside from Tom Baker and Mary Tamm and their interaction, very little in the serial received favorable reviews. Kroll certainly did not. The green paint for the Swampies’ makeup, specially ordered from Germany, was so difficult to remove that many actors had a green tinge to their skin for weeks.