The one where a lady karate-kicks a pantomime horse.
After having a wonderful time in the Death Zone, the Doctor decides that Tegan should see what future Earth will be like. The TARDIS lands in orbit of the planet Earth in the year 2084, where she immediately comes under attack by Sentinel Six, a killsat in charge of utterly destroying everyone without proper identification.
Mild panic ensues, but the Doctor is able to drop the TARDIS off somewhere mildly safer: an undersea military nuclear missile launch base known as Sea Base 4, also currently under threat of nuclear attack. The Cold War has somehow started up again by the year 2084, or maybe it's still going. The best we get from the Doctor is that "two power blocs" are engaged in a Cold War of sorts, but the entire story remains deliberately vague about it all.
As the Doctor explains this, we get introduced to our Red Shirt Army of the week, unisex eyeshadow and all. The "Them" bloc is acting up again, and the brave crew snap into action, as their computers say it's time to prepare the nukes. Well, maybe. As Commander Vorshak explains to his men, it could be real or it could all be a drill: they won't know until it's time to press that button. As a further failsafe, they also need someone lashed up to the machine itself to make sure the nukes can be launched by giving the final authorization. Cue Ensign Maddox, who's actually the inexperienced backup of someone unimportant who died offscreen. Maddox never really thought he'd be needed, as he was just there to observe, so he freaks out a little at the pressure despite Commander Vorshak telling him he's doing a great job... and it turns out it was all a drill, anyway.
Meanwhile, old Doctor Who enemies the Silurians and their sea-cousins the Sea-Devils are waking up from a long rest. They're now also calling themselves "Silurians" and "Sea Devils", despite both those terms having been made up by the Doctor (and despite "Silurian" being technically wrong anyway, as the Doctor had already pointed out years before). One of the Silurians, Icthar, is in charge as one of the remaining survivors from the last time the Doctor ran into them. However, time has not been kind to Icthar, or the others in his charge, as the Silurians now mumble incoherently with a little flashing light to tell us who is talking to whom. The Sea Devils, meanwhile, whisper menacingly... while seemingly bobbing about with their heads tilted to one side. (As Janet Fielding says in the commentary, it always looks like they're going "...you wha?") The Silurians and Sea-Devils have plans and these plans involve Sea Base 4.
Ensign Maddox, suffering from a panic attack, goes to the medical bay to lie down for a while. However, Senior Officer Nilson and Doctor Solow are up to no good, deciding to take advantage of Ensign Maddox's special computer brain to brainwash him. Because Nilson works for the "Them" Bloc! Doctor Solow follows along with this... because, presumably, she's also with the "Them" Bloc. Maddox is reprogrammed to sabotage the systems of Sea Base 4, but it's unknown if Nilson and Solow are trying to either capture the base or start the third (fourth? fifth?) World War.
Finally involved in the story again, the Doctor and his companions decide to meander around the top secret undersea nuclear missile base for no reason, exploring the area and getting a feel for the future. Since they landed in a storage closet, the Doctor decides that pointing out the barrels of hexachromite gas, which is a gas that's lethal to all marine and reptile life, would be a cool fact of the day for his companions to know. And then they're immediately chased around by guards, because Turlough accidentally sets off some alarms. While trying to provide a distraction by intentionally triggering the nukes, the Doctor is shoved into a giant pool of water. Inside the reactor everyone was just running through. Tegan frets about the Doctor for about two seconds, after which Turlough decides that the Doctor's dead because "HE'S DROWNED." Tegan and Turlough are then quickly captured by the soldiers of the "Us" Bloc, and the Doctor Who franchise ends in the death of the Doctor.
But not for long! Because it turns out the Doctor has survived! (And Davison is freezing. The water was supposed to be allowed to "warm up" over a few nights, but the ridiculously reduced schedule left them filming within minutes of filling up the reactor pool.) The Doctor somehow survives the intense radiation and swims (dressed) his way out of the reactor, taking the time to borrow another guard's outfit. And then make a fart joke. But not before deciding to overload Sea Base 4's reactor to "give them something to do".
The base crew is less than amused; also, Maddox seemingly going berserk and dying after taking out several important missile-launching computers doesn't help matters. But the Silurians attacking Sea Base 4 en masse really pushes the base into panic mode. The Doctor charges onto the bridge of Sea Base 4, and promptly realizes what's about to happen. He quickly explains to the crew of Sea Base 4 who's about to attack them all, and that they need to get ready for the worst: a Silurian weapon called The Myrka. The large, lumbering beast rips through Airlock-1 like it was badly supported plastic. This bio-engineered weapon is so fearsome that humans run up to be embraced by its deadly hugs and random spurts of electricity. Meanwhile, a contingent of Sea Devils, resplendent in their Samurai armor, begin shooting circles of light at soldiers from Airlock-5. The Doctor is able to destroy the Myrka with a bright blast of UV Light, but not before it kills Doctor Solow in a remarkable display of hand-to-hand prowess that was never ever rolled out before the public as a valid reason to kill off the show.
It doesn't take long for the Silurians to take over Sea Base 4, and they begin repairing all the damage done by the saboteurs, so they can launch the missiles themselves and begin World War Whatever. Don't bother asking why they didn't just sit and wait for mankind to do the job for them, otherwise we wouldn't have an episode. The Doctor does his best to argue that everyone should just get along, but Icthar will have none of it. After all, some jerk also named the Doctor promised that last time, and his people all wound up dead. Also, it's time to launch the missiles, destroy the Humans, and generally rule the world.
The Doctor escapes from the bridge along with a few of the humans and stumbles across the hexachromite gas mentioned earlier. Quickly running out of time, the Doctor realises that he'll have to use the hexachromite gas to flood Sea Base 4 and kill off every Silurian and Sea Devil. He goes into a bit of an existential crisis at having to kill so many people to save even more people, and commands Tegan and Turlough to tend to the gruesomely melting Silurians, while he tries a second option. He hooks up his brain to the Wetware CPU — which leaves him nearly catatonic with pain, his skin fried, his mind invaded in the worst possible way, and with everyone around him (except Team TARDIS) dead anyway because they just couldn't stop killing each other.
But hey, no World War.
This serial was a nightmare behind the scenes. As production was starting up, then-Prime Minister Margaret Thatcher announced a Parliamentary election, which resulted in the production for this serial losing two weeks to election coverage. Rather than cancel the serial and funnel the leftover money into the other productions that season, it was decided to damn the torpedoes and push ahead anyhow. Many have said this was a mistake, in retrospect.
The paint on The Myrka hadn't even yet dried before the scenes involving it were filmed, which got a bit silly when the green paint of the costume rubbed off on just about everything. The lighting, which the script called for being lowered to "dark" levels, was instead cranked up to maximum brightness. Because someone upstairs figured the elderly would call in, complaining the TV was on the blink.
Peter Davison and Janet Fielding both announced their intent to leave the series during the production of this serial. It is entirely possible that, had this serial not been done, Davison could have stayed on for another season (although Janet Fielding's contract may not have been renewed, as she had been on for three years by this point).
Finally, "Warriors of the Deep" is noted for being the last appearance of the Fifth Doctor's "first" outfit. The Doctor changes pants into something more creamy-pink and adds a stripe to his sweater for the next serial.
This episode of Doctor Who contains examples of the following 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: Turlough and some humans of the week go spelunking through some to escape some Sea-Devils.
- Attack Drone: Space satellite Sentinel Six
- Attack Its Weak Point: Averted, none of the soldiers try shooting the Sea Devils in their exposed face. They just continue blasting away at their blaster proof armour.
- Aw, Look! They Really Do Love Each Other: Downplayed, but Tegan and Turlough are much more caring toward one another than they have ever been in the past. Turlough breaks in front of the armed, hostile guards when Tegan is dragged into the room to check that she is all right, and they have another tender moment when they are reunited after Tegan has been told Turlough is dead.
- Big-Bad Ensemble: Icthar and Nilson.
- 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.
- Divided We Fall: The spies for the other human bloc use the alien attack to cover up their own activities.
- Downer Ending: The story concludes with massive character slaughter on the magnitude of a Shakespearean tragedy and, famously, ends with the line "There should have been another way."
- Eternal Recurrence: Humanity has once again divided into Eastern and Western power blocs with the power to destroy the world.
- Everything's Better with Samurai: The Sea Devil taskforce Elite Group One armor were similar to samurai (especially the helmet.)
- Exty Years from Now: The story is set hundred years in the future.
- The Great Politics Mess-Up: Averted since we're never explicitly told much of anything about either of the power blocs, just that there's two of them, which was a deliberate decision to avoid dating the politics.
- 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: The Doctor knocks out an unwitting guard before taking his outfit to better hide in plain sight. And then discards the plastic helmet that all the others wear.
- My God, What Have I Done?: The Doctor's summation at the end:"There should have been another way".
- Nice Job Breaking It, Hero!: One of the many times in Peter Davison's run where the Third option really didn't pan out well.
- No One Gets Left Behind: The Doctor invokes this when Tegan gets trapped below a bulkhead, in the path of the oncoming Myrka.
- "Not Important to This Episode" Camp: Kamelion doesn't appear at all.
- 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.
- Red Scare: Pretty much in tradition with the two previous stories, really.
- Red Shirt: Subverted, the only member of the supporting cast to survive, Bulic, wears a red shirt throughout.
- 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.
- Ripped from the Headlines: The story was written as an allegory for The Cold War.
- Samurai: The Sea Devils' armor invokes this image.
- Sequel Episode: To "The Silurians" and "The Sea Devils".
- Self-Serving Memory: Icthar tries to justify wiping out humanity by claiming that between them the Silurians and Sea Devils twice tried to forge alliances with the human race, but were "treacherously attacked" on both occasions. In actuality, only the lead Silurian in their debut story favoured trying to set up an alliance and was quickly killed by his subordinates when they refused to go along with the plan. Icthar's statement is a little more accurate to what happened in the Sea Devils' first appearance, though leaves out the fact that the Doctor was never able to communicate the offer of peace before the humans launched their attack, and the Sea Devils refused an offer of mediation afterwards.
- Sexy Soaked Shirt: Peter Davison swims! Dressed.
- Styrofoam Rocks: Styrofoam Bulkhead. To the point where it is openly flexing and rippling when touched.
- Take a Third Option: The Doctor tries this but fails.
- 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.
- Too Dumb to Live: Solow attempts to karate kick the Myrka. It does not end well.
- Ultraterrestrials: Both the Silurians and Sea Devils, as seen in previous serials.
- Underwater Base: The serial's setting is a futuristic example.
- What A Senseless Waste Of Human And Earth Reptile Life: The final line.
- What Measure Is a Non-Human?: The Doctor gets upset at the humans killing Silurians and Sea Devils, even though the reptiles attacked first. Every story with Silurians tries to avert this.
- 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. It was this footage, amongst others, that was later to be presented by BBC executives who wanted to axe Doctor Who.
- There's an interesting sequence where the Doctor defeats a whole platoon of guards in hand-to-hand combat while constantly making it look like he has no idea how to fight. Why the Doctor is projecting Obfuscating Stupidity while being in the process of kicking arse is unclear.
- The X of Y: One of many to use this naming scheme in Doctor Who.