In 1983, a Russian submarine plies the waters of the North Pole. A professor aboard the sub has dug up some mysterious creature from the ice, with plans to thaw it out in Moscow. A crewman gets ants in his pants and decides he can't wait that long, and ends up unleashing a bipedal armadillo-lizard thing that longtime fans probably recognized immediately. Mayhem ensues and the sub ends up sinking. Just when it looks like the Russians are done for, the TARDIS materializes and out pop the Doctor and Clara, having really missed the mark when they were looking for Las Vegas. The Doctor gains a modicum of the Captain's trust when he keeps the sub from sinking further, but the TARDIS decides to hightail it because screw this crap!
The armadillo-lizard thing introduces himself as Skaldak the Martian Ice Warrior, whom the Doctor recalls is a great hero amongst his people. He and the other Martians became cyborgs after conditions on the planet necessitated it, hence the armadillo-lizard shell. Skaldak is somewhat dismayed to learn that he's been frozen for 5,000 years. When a crewman zaps him with a cattle prod, he sends out a distress call to his fellow warriors. He decides that his entire species must be dead since they didn't immediately answer his call, and goes on a murderous rampage. To do this, he leaves behind his cyborg shell and skitters around the sub as a pair of gooey green rubber gloves and glowing red eyes.
As Skaldak sneaks around the sub, he learns of things like "cold war" and "mutually assured destruction." He figures that since all his people are dead and he's got nothing to live for anyway, he should just start a nuclear war between the Russia and everyone else. His motives are... kind of lame. The Doctor and Clara try to talk him out of it, with some variation of "won't somebody please think of the children?!" which serves to stall him until the other Ice Warriors arrive to teleport him onto their ship.
The day thus saved, the Doctor explains that the TARDIS (in)conveniently disappeared because it sensed danger or something, and it's part of some security system. The TARDIS is safe and unharmed and currently parked at the other end of the planet at the South Pole. The Doctor's like, "Can you give us a lift?" The Russian captain laughs and everyone laughs because the episode suddenly turned into a sitcom. Stay tuned for the full weecap.
An English submarine dives into cold black water. Sailors scamper to and fro as an English voice says, "The signal is genuine!" This is the Captain's cue. He gives the order to launch nuclear weapons, also in English. "Moscow confirms launch sequence!" someone says, because the sub is actually Russian and so is her crew. The TARDIS is so powerful that it translates for us several minutes before it even arrives! The Captain reaches out to push the button that will plunge the globe into nuclear war. Suddenly, David Warner strolls onto the bridge, singing "Vienna" by Ultravox. Everyone's like, "What's David Warner doing on the bridge singing 'Vienna' by Ultravox?" He notices them staring at him and asks, "Did I interrupt something?" The Captain explains, "We were about to blow up the world, Professor." The Captain calls off the drill. His first officer (I guess) flounces off because he wants to keep running drills. "Did you have your specimen stowed okay?" the Captain asks. "Yes, Piotr's looking after it," says the Professor, except he pronounces it "Peter" despite how the CC spells it.
They talk about how the specimen is probably a mammoth, even though as the camera pans down to the hold, it's clear that the shape within the block of ice is about as elephantine as the crew are Russian. Piotr doesn't want to wait until they get to Moscow and decides to take a torch to the ice. There is no reason for this to happen except that the story needed it to happen. I'm surprised that Mark Gatiss wrote this episode, considering I generally find his work much more skillful. Anyway, Piotr thaws the ice just enough that an armored fist is able to punch through from within and grab him by the throat. Good things come to those who wait. Death and destruction come to those who don't.
By the time the opening credits are over, the sub is in quite a state of panic. The creature freed from the ice has been causing so much havoc that the ship is now sinking fast and taking on water. The TARDIS whomp-whomp-whomps onto the bridge and out pops the Doctor in Elvis sunglasses. "Viva Las Vegas!" he shouts an instant before he and Clara tumble out. They should have made a left turn at Albuquerque. The Captain is quite aghast at their sudden arrival, but has more pressing matters at hand. The Doctor quickly assesses the situation, babbles something about turbines and momentum, and suggests the Captain maneuver the sub laterally. His sonic screwdriver has picked up an underwater ridge nearby. They can land on it! Or break apart on it! The sub groans under the increasing pressure. The Captain gives the order to follow the intruder's suggestion, much to his first officer's dismay.