Romana: As what?
The Doctor: Oh, I don't know... a wise and wonderful person who wants to help. Don't exaggerate.
The search for the Key to Time takes the Doctor and Romana to the third moon of Delta Magna, a marsh-covered world where the indigenous tribesfolk worship a giant squid-god called Kroll. K-9 can't come along (it's too wet, and his actor is busy playing a mook). Things are complicated by the presence of a mining operation from Delta Magna, which is stealing the natural resources and angering the locals — and then it turns out that there really is a Kroll, the mining operation has woken him up, and he's more than a little grumpy. Kroll's really just an overgrown squid brought over by the colonists as a little one, but that doesn't make him less dangerous.
The head of the mining operation, Thawn, decides to blast Kroll with rockets, but the Doctor disables them just in time for the locals to mount an attack on the refinery and kill him. The gizmo the Doctor has been using to track the Key to Time shows it's Kroll — or maybe it's just inside Kroll... Turns out Squidzilla has eaten a local relic which is the Key segment, and the Doctor uses his gizmo to transform it back again, which has the handy effect of turning one giant squid into lots of normal-sized ones. They retrieve the segment, prevent the whole rig from blowing up as an aftereffect, and leave the one remaining villain to sort things out with the tribesfolk on his own.
Robert Holmes has called this his least favourite of the ''Who'' stories he wrote, describing it as rather dull. He was never a fan of "big scary monster" stories, and so was uneasy with the premise he was given, as well as the way the script was executed. It also suffers from budgetary issues, even by Who standards of the time.
- Absentee Actor: Sort of. K9 is absent from this story, as the prop wouldn't work on the swamp location. His voice actor, John Leeson, plays Dugeen.
- Arms Dealer: Rohm Dutt, a smuggler from Delta Magna who's selling firearms to the local tribe.
- Big Bad Ensemble: On the one side there's Thawn, who wants nothing more than to wipe out the Swampies, and is quite willing to kill anyone who gets in his way. On the other side is Ranquin, who similarly wants to get rid of the humans, while feeding any dissenters to Kroll. And then there's Kroll itself, which doesn't really care about any of this and is perfectly happy to just eat anyone who gets within range.
- Book-Ends: A subtle one. Kroll came into being by eating a disguised Key to Time segment, after which it started growing and devoured a Swampie high priest. Its final act before being reduced to the segment and hundreds of baby giant squids is to consume the current high priest, Ranquin.
- Cassandra Truth: During the final episode the Swampies gradually begin to realize that Kroll isn't really a god at all, just an oversized, mindless beast. Ranquin, however, holds onto his belief until the bitter end, resulting in him becoming Kroll-chow.
- Chained to a Rock: The Doctor and Romana, by the local tribe.
- Chromosome Casting: Romana is the only female character in this story.
- Crapsack World: it's an equal-opportunity-misanthropy depiction of colonialism where the colonists are all murderous racists or ineffectual wets, but the indigenous people are bloody-minded, backwards, sadistic, and worship a mindless monster just because it's big and scary. The ending gives the strong impression that the Doctor has absolutely no confidence in his standard "and now you must learn to live in peace together" speech and that the Swampies are either going to execute the last surviving colonist as soon as team TARDIS is out of sight, or start worshipping him for helping to kill the old god.
- Does This Remind You of Anything?: The setting resembles any number of situations in Earth history where relatively primitive nations are exploited by their more civilised neighbours. Except for the bit about the giant squid.
- Family-Unfriendly Death: Thawn is impaled by a spear, complete with blood spewing through his fingers.
- Fantastic Racism: Between the humans and the Swampies, to varying degrees. There's Thawn and Ranquin, who are very much the "genocidal lunatic" variety, Dugeen, who actually doesn't mind the Swampies at all, and varying shades in-between the two extremes.
- Fantastic Slurs: "Swampy" and "Dryfoot" are what the miners call the locals and the locals call the miners, respectively.
- The Fundamentalist: Ranquin. With basically everything that happens, he scrambles to find some way to interpret it as Kroll confirming his worldview.
- Gallows Humor: Ranquin asks the Doctor why he's so inquisitive when "You Have No Chance to Survive":Doctor: (bound to a tortuous Death Trap that will slowly stretch him out) "I like to get things straightened out."
- Giant Animal Worship: a giant squid is worshiped as a god. It's so big its waste is refined to fuel. It swallowed a segment of the Key To Time.
- Giant Squid: Kroll is this trope combined with Attack of the 50-Foot Whatever, and the show's largest-ever monster up until that point.
- Human Aliens: The miners look just like (and may be descended from) Earth people. The locals also look a lot like Earth people, apart from being green.
- Human Sacrifice: The Swampies first try to sacrifice Romana to Kroll, then The Doctor, Romana and Rohm-Dutt in one go.
- Hypnotic Eyes: The Doctor is unable to hypnotise someone into releasing him.
- I Meant to Do That: Ranquin insists his people left their original planet because the high priestprophecized the ecological disaster the colonist would bring about. The Doctor states they were forcefully relocated by the colonists.
- Low Culture, High Tech: Thawn hires an arms dealer to sell defective guns to the natives as an excuse to get rid of them.
- Make Me Wanna Shout: Used by the Doctor to shatter a particularly tasteless window.
- Man in a Rubber Suit: The fake Kroll who tries to kill Romana is this even within the story.
- No-Sell: The Doctor tries to hypnotize Ranquin to no avail, apparently due to his "narrow little eyes".
- Occupiers Out of Our Country: The Swampies aren't overly keen on the idea of sharing their homeworld with humans, even the small crew of the refinery.
- Oddly Small Organization: the Swampies' race apparently consists entirely of one village as it is stated in-story that destroying it would end their entire civilization.
- Outgrown Such Silly Superstitions: Romana is very dismissive of Kroll and his titular power.
- Pretext for War: Thawn wants to wipe out the Swampies, but the Swampies' (off-screen) supporters, the Sons of Earth, are non-violent. So he covertly gets Rohm-Dutt to supply them with defective weapons so they'll attack and give him a reason to kill them.
- Red Herring: Mensch is set up to be a potential Swampie Double Agent, with both the humans and Swampies being unsure of where his true loyalties are. Ultimately it doesn't really matter, as he becomes Kroll's first victim.
- Shout-Out: The Doctor's remark "Will there be strawberry jam for tea?" is a quotation from the short story "The Lumber Room" by Saki.
- The Skeptic: Varlik clearly doesn't buy into the Kroll myth as much as the other Swampies, and in the latter stages begins openly questioning Ranquin.
- Smug Snake: Many people might plan to wipe out an intelligent species, only Thawn would do it for a reason as petty as expanding a business (which isn't even his business, he seems to be middle management at best). His plan to sell the Swampies faulty guns and then shoot them in self-defence falls apart because a giant squid starts killing everyone indiscriminately and his staff aren't as sociopathic as him, even giving him funny looks when he starts chuckling about how all the people he doesn't like are going to die. Even the Doctor only sees him as someone to snark at. When he gets a spear in the chest, no-one gives him a second look.
- The X of Y: One of three Key to Time stories (the others being "The Stones of Blood" and "The Androids of Tara") to follow this format. Originally the title had a double meaning, referring both to both the power that Ranquin had over the Swampie tribe thanks to the Kroll legend, and the real Kroll being a giant building-wrecking behemoth, though it's much less apparent in the finished story.
- You Have Failed Me: Ranquin interprets Kroll's destruction of the Swampie village as an example of this, for failing to deliver the Doctor and Romana. He sticks with this belief even after the others point out that it was because of Kroll that they weren't able to capture the two Time Lords.