The Doctor and Clara find themselves dealing with Cold War tensions when the TARDIS lands aboard a disabled Soviet ballistic submarine. The Doctor isn’t the only alien being on board however – while engaged in undersea drilling, Professor Grisenko thought he’d found a mammoth and brought the creature on the boat encased in ice. It isn’t a mammoth but an Ice Warrior, who has been lying dormant for 5000 years. He identifies himself as Grand Marshal Skaldak but when he cannot contact others of his kind, he assumes he is the last of his race. He now has every intention of starting a thermonuclear war between the superpowers and destroy Earth.