A Soviet submarine is under the North Pole in 1983, height of the Cold War. In the cargo hold, a sailor causes a block of ice to thaw. Professor Grisenko (David Warner) believes it contains a mammoth, but it is an Ice Warrior, which attacks the Professor. The submarine begins to sink, the Ice Warrior running amok. The TARDIS shows up at this point and the Doctor (Matt Smith) and new companion Clara (Jenna-Louise Coleman) tumble out. The Doctor convinces Captain Zhukov (Liam Cunningham) to land on a ridge.
The TARDIS dematerializes by itself, trapping everyone on board. The Doctor convinces the crew not to attack the Ice Warrior, Grand Marshal Skaldak (Body by Spencer Wilding, voice by Nicholas Briggs). But Executive Officer Stepashin (Tobias Menzies) knocks out Skaldak. Knowing he will seek revenge for the dishonour, the Doctor and the crew chain him up.
Clara talks to him in the Doctor’s words to try to calm him. They tell him he was encased in ice for 5000 years. Skaldak mourns his daughter and his people. The Doctor tells him the Ice Warriors still live, and not just on Earth and Mars. Skaldak does not believe it, but thinking he is the last of his kind, he kills three members of the crew.
Having learned about the Cold War and Mutually Assured Destruction, Skaldak decides to use the submarine’s nuclear missiles to destroy humanity. On the bridge, he connects himself to the missile guidance systems and activates the missiles. The submarine is rocked by a tractor beam and hauled to the surface by the Ice Warriors, who have heard Skaldak’s distress call. Skaldak is beamed away and deactivates the missiles. Unfortunately, the TARDIS is at the South Pole.
The Ice Warriors had not appeared since 1974. Their somewhat goofy armour was redesigned to look more menacing. Nicholas Briggs, who voiced all the Daleks and Cybermen, provided the wonderful serpent-like voice of Skaldak. The episode was highly thought of, called tense and claustrophobic. David Warner is known for many roles, particularly for Star Trek. He was considered for the Doctor at one time, as was Liam Cunnigham, best known these days for Game of Thrones. The crew were named for characters in Russian literature, war, and politics, and the submarine was The Firebird.
The cast was constantly doused with water and were dripping wet most of the time. The Cold War really was dangerous. More than once, missiles were almost launched, and the shooting down of a Korean airliner killed 269 people. Clara was different from other companions; in that she was capable of doing what she was told. When the Doctor tries to talk down Skaldak by appealing to his sense of honour and history, she asks him to think of his daughter and all the countless innocents who would die, pointing out just why the Doctor needs a human companion.