∇
troperville

The Doctor, Tegan and Turlough wind up in a military undersea base c.2084 and the Cold War is at it again. Or still at it, possibly, as this was made in 1984. The trio explore this base, and the companions are quickly captured so the Doctor can rescue them later on. As it turns out, this base is the futuristic equivalent of an underwater nuclear missile silo - with Cold-War-ish tensions rising quickly and a spy within the base trying to either capture the base or start the third (fourth? fifth?) world war.

Eventually, it's discovered that a group of indigenous intelligent reptoids (the Silurians and the Sea Devils) are invading this base to stop the humans from making the next world war so that... the ~~aliens~~ indigenous reptoids themselves can start up the next world war. Don't bother asking why they didn't just sit and wait, it's a perfect excuse for them to whip out the dreaded Myrka - an bio-engineered weapon so fearsome that humans run up to be embraced by its deadly hugs and random spurts of electricity. Fed up with no one listening to him, the Doctor essentially goes commando on the whole base and (not through lack of trying to save the day) every single person on the base who wasn't in the TARDIS at the start of the adventure (except one) winds up dead.

But hey, no world war. At least he didn't break everything... er...

Tropes

Action Girl: The episode tries to set up Preston and Solow as examples of this. Unfortunately, Preston never really gets the chance to do much of anything, and the less said about Solow's big action scene with the Myrka, the better.
Air Vent Passageway
Blinded by the Light: The Myrka is killed by an intense ray of ultraviolet light aimed at its eyes.
Borrowed Catchphrase: The Fifth Doctor says "When I say run, run. RUN!", which was a catchphrase of the Second Doctor.
Chekhov's Armory: A rather obvious one in the hexachromite gas, which is lethal to marine and reptile life.
Deadly Gas: Just not, in this case, deadly to mammals.
- In a real-life version, the two actors in the Myrka costume were breathing in fumes from the wet paint on the costume pretty much the whole time they were in it.
Dirty Coward: Turlough
Downer Ending
Everything's Better with Samurai: the Sea Devil taskforce Elite Group One armor were similar to samurai (especially the helmet.)
Exty Years from Now
Franchise Original Sin: The aforementioned fight scene with Solow and the Myrka was presented by executives who wanted Doctor Who axed to BBC 1 Controller Michael Grade, and it worked... for 18 months. invoked
The Great Politics Mess-Up: Maybe averted since we're never explicitly told much of anything about either of the power blocs, just that there's two of them. The novel pretty heavily implies that the power blocs originate from the Cold War, however.
Humans Are The Real Monsters:
The Doctor: I sometimes wonder why I like the people of this miserable planet so much.
I Did What I Had to Do: but "there should have been another way."
Kill 'em All: The surviving characters, even including the regulars, can be counted on one hand.
Mugged for Disguise
No One Gets Left Behind: The Doctor invokes this when Tegan gets trapped below a bulkhead, in the path of the oncoming Myrka.
Pantomime Animal: The Myrka is played by two dudes (actually the same guys as in Rentaghost) in an unfinished costume wet with paint. It... doesn't get pulled off well.
Reasonable Authority Figure: Commander Vorshak
Reptiles Are Abhorrent: The Doctor tries to convince the Sea Base crew otherwise. Problem is, this line of reasoning doesn't work when said reptiles are killing everyone.
Samurai: The Sea Devils' armor invokes this image.
Styrofoam Bulkhead: To the point where it is openly flexing and rippling when touched.
Third Eye: Only this time, the Silurians don't use it as a weapon.
This Is Not a Drill: The computer only tells the people on the base whether or not a missile run is a drill after they've gone through all the motions that will launch their weapons of mass destruction if it is not.
Troubled Production: The production schedule lost two weeks due to an early Parliamentary election, which explains many of the serial's shortcomings. The other shortcomings stem from the set designer, who wanted everything brightly lit to show off his work, and the director, who wanted everything dark and moody for atmosphere, preceding a similar situation on Aliens by two years.
Ultraterrestrials
Underwater Base
Wet T-shirt Scene: Peter Davison swims!
What A Senseless Waste Of Human And Earth Reptile Life: The final line.
Wimp Fight: When the Myrka attacks one of the characters, she crosses her arms, twists to the side and performs some kind of half-hearted kick to the creature. It responds by trying to hug her to death.
The X of Y