This story often seems to get quite a bad press, but I actually have quite a fondness for it. It's written by the great Robert Holmes which is always a good start, and some elements of the plot are actually reused in the later (and much more highly regarded) Caves of Androzani. In terms of the Key to Time arc it's one of the better uses of the segment, having some sort of back story about the symbol being swallowed by a squid and it causing it to grow to enourmous size, as opposed to some of the other stories where the segment is just hidden somewhere without any explanation as to why it's disguised as it is. The location filming is fantastic, with the suffolk marshes really convincing as an alien planet, and when you add in the storms, the green-skinned natives, their mud huts and Kroll itself you get a really convincing impression of a damp alien world. Besides, how can you not love story that has actors painted bright green jumping around in a squelchy swamp in southern England? The actors treat their roles so seriously that despite this ludicrous notion, you can't help but be drawn into the whole thing.
Then we have the refinery crew. I think it's quite well known that Philip Madoc was originally offered the role of Thawn, but for some reason it ended up going to Neil McCarthy instead, and Madoc was rather disgruntled about it. This clearly shows in Madoc's performance, as he plays Fenner with a constant air of bored irritation but this actually dosen't really matter as I think alot of people would probably behave like Fenner if they were couped up inside that dull metal refinery with only 3 other blokes to get on each others nerves. The fearsome-looking Neil McCarthy is actually pretty good as Thawn, a man with a xenophobic hatred of Swampies who gets progressivley more unstable as the story progresses, and voice-of-K9 John Leeson gives a respectable perforamnce as nice guy Dugeen. The other bloke in the refinery is so forgettable though that I've forgotten his name.
Tom is on top form in this story, at his wacky alien best, (I love his duck badges and green fishing waders!). and the model work for Kroll is absolutely superb, particularly in the scenes where it's attacking the refinery. The stubborness of John Abineris Ranquin (another fine performance) leads inevitably to his rather amusing demise by tentacle being sucked into a pipe, and the base-under-siege finale is classic Doctor Who.
For me, The Power of Kroll exemplifies what Doctor Who meant to me as a child - alien planets, big green tentacled monsters, aliens, bases under siege and Tom Baker at his otherworldy, eccentric best. It may not be the best story ever made, but it looks good, it's exciting (I forgot to mention the excellent death-by creeper scene and Romana's sacrifice!) amusing (sometimes unintentionally) and never dull. I wonder what became of Fenner at the end - maybe he decided to go native with the Swampies and is now prancing around in a leather jock-strap chanting"Kroll!Kroll! Kroll!"