Doctor: Matt Smith (Eleventh Doctor)
Jenna-Louise Coleman (Clara Oswald)
Liam Cunningham – Captain Zhukov
David Warner – Professor Grisenko
Tobias Menzies – Lieutenant Stepashin
Josh O'Connor – Piotr
James Norton – Onegin
Charlie Anson – Belevich
Spencer Wilding – Skaldak
Nicholas Briggs – Voice of Skaldak
Scott Stevenson – Restraining submariner
Directed by Douglas Mackinnon
Written by Mark Gatiss
Produced by Marcus Wilson
Incidental music composer Murray Gold
Series Series 7
Length 45 minutes
Originally broadcast 13 April 2013
A Soviet submarine is sailing near the North Pole in 1983 during the Cold War. A nuclear weapons launch drill they are running is interrupted by Professor Grisenko (David Warner). In the submarine's cargo hold a sailor prematurely begins to defrost a block of ice that Grisenko believes contains a frozen mammoth, and is attacked by what is inside.
The submarine begins sinking as the creature in the ice escapes and runs amok. The TARDIS, which originally was on a course set for Las Vegas, materialises inside the submarine and the Doctor and Clara tumble out. The Doctor convinces Captain Zhukov (Liam Cunningham) to maneuver the submarine to the side, landing it safely and preventing it from imploding. During this the TARDIS inexplicably dematerialises. They then encounter the escaped Ice Warrior, Grand Marshal Skaldak (Spencer Wilding, voiced by Nicholas Briggs). The Doctor encourages the crew to be peaceful, but the ship's executive officer Stepashin (Tobias Menzies) knocks out Skaldak with a cattle prod, suspecting that he is not an alien but part of a Western plot. Knowing that Skaldak will desire vengeance for being attacked, the Doctor and the crew chain Skaldak, who sends out a distress call for others of his race to find him.
Clara volunteers to speak to Skaldak to try and calm him, relaying the Doctor's words to Skaldak, though he knows that the Doctor is listening. The Doctor informs Skaldak he was encased in the ice for 5000 years. Skaldak laments the loss of his daughter and his people; however, the Doctor tries to comfort him with the knowledge that the Ice Warriors still live, just not on Earth or Mars. Skaldak escapes from his armour - an act of great dishonour - and stops broadcasting the signal to the other Ice Warriors because he believes himself to be the only one of his kind left. The Doctor surmises that, thinking himself to be the last of his kind, Skaldak has nothing left to lose. He manages to grab and kill three members of the crew.
Having learnt of the ongoing Cold War and the doctrine of mutually assured destruction from Stepashin, Skaldak plans to use the submarine's nuclear missiles to provoke a global thermonuclear war and destroy humanity as revenge for the humans attacking him - under the Martian code humanity as a whole has declared war on the Ice Warrior race by assaulting him. Reaching the bridge, he is able to connect himself to the sub's missile guidance systems and activate the missiles. The Doctor and Clara are attempting to persuade Skaldak to show mercy when the sub is rocked by a tractor beam from above: the Ice Warriors have arrived over the site of the submarine's undersea grounding, and they haul it to the surface. Skaldak is beamed aboard the Ice Warriors' spaceship, though the missile launch system is still active. Showing mercy, Skaldak deactivates the missiles remotely. When the Doctor's sonic screwdriver informs him the TARDIS has reappeared, he informs Clara that it had "relocated" to the South Pole automatically as part of the H.A.D.S. ("Hostile Action Displacement System"), which the Doctor admits he reactivated by accident while fiddling with the controls. As they are at the North Pole, an embarrassed Doctor asks Zhukov for a lift to get the TARDIS before saluting the Ice Warrior ship as it leaves.