Doctor Who/Recap/S13/E06 The Seeds of Doom

Harrison Chase: "What do you do for an encore, Doctor?"
The Doctor: "I win."

"You and your kind are nothing but parasites. You're dependent upon us for the air you breathe and the food you eat. We have only one use for you..."

—The Krynoid, speaking through Harrison Chase

The TARDIS arrives in England as two large alien seed-pods are found in the Antarctic permafrost. Helicoptered out to the Antarctic base to see for himself, the Doctor identifies them as Krynoids - a particularly rapacious plant that will, if allowed to germinate, consume all life on Earth.

One of the scientists, Winlett, is infected by the pod and begins to turn into a Krynoid himself, but is destroyed by a bomb set by two mercenaries called Scorby and Keeler, who steal the other pod on behalf of their employer, plant collector Harrison Chase. Chase takes Sarah hostage and plans to open the pod in her presence, which will infect her. The Doctor rescues Sarah and it is Keeler who is infected. This doesn't bother Chase much, and he begins to feed Keeler raw meat to accelerate the transformation.

The Keelernoid grows bigger and bigger, eventually beginning to take over Chase's mansion, while the mad collector decides to turn the Doctor into compost to feed it - but ends up falling into the crusher himself, while the Krynoid is bombed by the RAF.

Not to be confused with "The Seeds of Death".

Tropes

BBC Quarry: As Antarctica with a white paint job. A different part of the same quarry was then used as an actual quarry for the fight scene between the Doctor and Chase's chauffeur.
Body Horror: There's no other way to describe the Krynoid transformation.
Bond Villain Stupidity: Scorby, all the time.
Conveyor Belt O' Doom: Leading into Chase's compost machine.
Doomy Dooms of Doom
Dressing as the Enemy: The Doctor gains entry to Chase's mansion by stealing the hat and coat of the limo driver who tried to kill him and just driving through the front gate. Once he actually gets out of the car and the guards realize he's not the regular driver, the disguise fails. Hilarity Ensues.
Evil Is Not a Toy
Fate Worse Than Death: "It's more serious than death, Mr. Stevenson. He's changing form."
For Science!: Chase's original motivation until the Krynoids possessed him.
Frickin' Laser Beams: UNIT now has rather clunky and homemade-looking laser guns. It still does little more than annoy.
Garden of Evil: Chase's estate is turned into one of these as all the flora falls under the control of the Krynoid.
"I Know You're in There Somewhere" Fight: Averted by the first Krynoid, as the Doctor makes it clear that Winlett's consciousness was destroyed at a relatively early stage of the infection. Subverted by the second Krynoid, as it's revealed that the Krynoid did keep some of Keeler's personality... but only enough to enable it to speak English.
Karmic Death: Chase ends up shredded in his own human compost machine, after trying to drag the Doctor in. While the Doctor was trying to save him.
Kill Us Both: the Doctor ends the story by calling in an air strike on a building he's trapped in. He gets out in time.
Little Old Lady Investigates: Amelia Ducat.
Man-Eating Plant
Minion with an F In Evil: Keeler is only a botanist, sure, but you'd think Chase would've sent someone more ruthless to assist Scorby in Antarctica. Keeler objects to everything Scorby does and even initially refuses to use a gun. When he finally accepts a gun the poor guy is such a nervous wreck it's amazing he doesn't accidentally shoot himself.
Moral Guardians: one of a number of Who stories from the era to be specifically attacked by Mary Whitehouse. In this case, she complained about "strangulation by obscene vegetable matter" and the possibility of children imitating the scene where Scorby makes a petrol bomb.
Mysterious Antarctica
Not My Driver
Obstructive Bureaucrat: The Doctor demonstrates how to get their co-operation.

"If that Krynoid spawns it will mean the end of everything, even your pension!"

Plant Aliens
Put on a Bus: This serial marked the last appearance of UNIT in the series for a long time. Except for a minor appearance in The Five Doctors, there wouldn't be another UNIT story until Battlefield 13 years later.
Pragmatic Villainy: When Amelia, who Chase tried to welsh on earlier, comes around demanding her money for the painting she did, Chase not only pays what he owes but doubles it in order to get her out of his house as quickly as possible. See also Rewarded as a Traitor Deserves below.
Private Military Contractors: Scorby. Chase's house is also guarded by his private security force armed with Uzis.
Recycled Script: Noticeably recycled from a episode of The Avengers (not by the same writer). Also strong similarities to The Quatermass Experiment.
Redemption Equals Death: Dunbar.
Rewarded as a Traitor Deserves: Completely averted. Chase actually pays Dunbar, his spy in the World Ecology Bureau, the briefcase full of money he promised him rather than killing him.
Save the Villain: The Doctor tries to save Chase, who responds by trying to pull him into the compost machine with him. Big mistake, as it's difficult to hold onto someone while your legs are being crushed.
Screw the Rules, I Have Money
Screw This, I'm Outta Here: Most of Chase's guards wisely bail offscreen when things start getting rough, except for one who apparently wasn't fast enough and gets killed by some vines.
Sealed Evil in a Can
Self-Disposing Villain: See the Save The Villain example above.
Sissy Villain: Chase: "I could play all day in my green cathedral!"
Stylistic Suck: Chase's music.
Suspiciously Similar Substitute: Two this time: Sergeant Henderson for Benton and Major Beresford for The Brigadier.
Tap on the Head: Chase knocks Henderson out by bopping him lightly with a big wrench.
Utopia Justifies the Means: The Krynoid seems to have this as an instinct. Chase likewise feels this way when it comes to plants.
Was Once a Man: Winlett, then Keeler.
The X of Y