The monster that gets the Award for Biggest Monster in a Doctor Who episode turns up in the Season 16 story The Power of Kroll, with Tom Baker as the Fourth Doctor, Mary Tamm as Romana, and a fairly static, superimposed rubber cephalopod as Kroll. It aired between 23 December 1978 and 13 January 1979.
The story itself is a classic mix of tropes that involve an ecology disrupted by careless developers, indigenous exploitation and displacement, fanatical worship of a “natural” deity, capitalistic imperialism, native rebellion and giant monster destruction.
Writer Robert Holmes does a pretty good job with the script, Norman Stewart’s direction is servicable and as usual the acting is excellent, with even minor characters being well served by a group of experienced British actors. Some viewers have expressed disdain toward the costuming used for the “Swampies” (the original inhabitants of Delta Magna, now refugees from a capitalistic regime on the planet’s third moon), but it works within the context, provided a little discretionary tolerance is applied.
And though the huge tentacled Kroll is really too ambitious a creature for the available SFX budget, what we get does work on a simple conceptual level, despite size discrepancies between long shots of the monster and the incongruous smaller tentacles that attack on a personal level. Kroll clearly has a mass of tentacles that can attack from beneath the swamp:
Or through the ducts and piping of the offending refinery:
Though visually the concept doesn’t quite gel, Kroll is rather like the monstrous squid from Stephen Sommers’ Deep Rising — able to send long, smaller tentacles into narrow spaces in search of offending humans. In fact, now that I’ve mentioned it, there’s some similarity between the “faces” of the two monsters as well, though Kroll wins out in sheer bulk:
And the Deep Rising monster as revealed in the climactic scene:
All told, despite a somewhat rushed appearance and the dodgy, less-effective choreography of the Swampie’s religious ceremonies, The Power of Kroll is an enjoyable Doctor Who story — with the lead monster definitely one of its main attractions.