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We’re at the North Pole in 1983, aboard a Soviet submarine (though you wouldn’t know that from the British accents scattered about the control room). The unnamed captain is running nuclear-launch drills, which a goofy scientist interrupts by meandering about singing along to Ultravox. Meanwhile, in the depths of the vessel, a crewman is thawing out a mysterious block of ice that contains a creature the professor wants thawed back in Moscow. If I am ever stationed aboard a Soviet nuclear sub, I certainly hope I’ll have more sense than to start poking around mysterious cargo on the chance that it contains something awesome, like caviar or a troupe of frozen balalaika players. Because, most likely, it contains a creature bent on killing everything aboard the ship. Lo and behold, a monstrous arm explodes from the ice and begins to choke the crewman. Roll credits!
It turns out having a shootout with a giant monster-thing is no good for hull integrity, so when the Doctor arrives on the scene, the sub is already sinking. The Doctor and Clara roll out of the TARDIS thinking they’re headed for Las Vegas. The Doctor somehow thinks a sinking Soviet submarine is “better than Vegas,” and saves the vessel by moving it along to an underwater ridge. The thoroughly ungrateful Soviets search the Doctor, who is separated from a Barbie doll, a bit of twine, and his sonic screwdriver. The TARDIS flies away for no apparent reason, and Clara blacks out.
When she wakes, the Doctor is in an argument with the Captain. Before anyone can get shot, a man the size of a tank (with the armor to match) approaches from behind. The Doctor informs everyone that this is an Ice Warrior, a native of the planet Mars. We haven’t seen these dudes since the ’70s. This one has been encased in ice for 5,000 years, and he just so happens to be Grand Marshal Skaldak, the greatest Ice Warrior of all time. A crewman zaps Skaldak with a cattle prod, and the Doctor orders that he be locked up. When Rory waited for Amy 2,000 years, he came out of the ordeal relatively cheerful, but no such luck for Skaldak. He sends out a distress beacon to his brothers.
Meanwhile, the Doctor explains that the Ice Warriors are a militaristic people who live in their impenetrable survival suits. We set up that the Captain’s second-in-command is much more into violence than his superior, who’s willing to negotiate with the creature. Clara volunteers for the job of talking to Skaldak, while the Doctor tells her what to do via a headset. To everyone’s shock, this doesn’t go to well. Skaldak has left his armor, and is roaming around the submarine naked, killing and mutilating members of the crew.
The group splits up, and the war-monger lieutenant offers to make an “alliance” with the creature. He accidentally gives Skaldak the information he needs: If Skaldak can launch one nuclear missile, then the Americans will retaliate, and all of humanity will perish. It’s called “mutually assured destruction,” and the Ice Warrior is a big fan.
The nutty professor reunites the Doctor with his Barbie and his sonic screwdriver. He also tries to engage Clara in a sing-along of “Hungry Like the Wolf” while the Doctor tries to save the vessel, find the Ice Warrior and stop a nuclear war. Clara isn’t exactly doing much except holding a flashlight. It turns out that she feels pretty bad about not contributing, particularly about how poorly her conversation with the Ice Warrior went. After getting some moral support from the professor (who demands to know if Ultravox breaks up in the future), the two are set upon by the Ice Warrior. For the first time, we see him outside of his shell suit, reptilian face and all. He calls his suit to him using sonic technology, and promises to kill all humans.
The Doctor follows him to the bridge, where Skaldak is trying to launch a nuclear missile. The Doctor argues for mercy, but then threatens to blow up the sub before Skaldak can destroy Earth. It’s mutually assured destruction all over again! Clara appeals to his fatherly instincts, but before any decisions need to get made, the Martian Ice Warriors arrive in their sparkly purple ship, sonic-ing the whole sub to the surface. Skaldak gets beamed up, and tells his buddies to call off the nuclear launch (but not before we get treated to Clara Oswald’s version of “Hungry like the Wolf.” Meanwhile, the TARDIS is off in the South Pole, and everyone has a good laugh about how the heck the Doctor and Clara will get there.