The Doctor and Clara wanted to go to Las Vegas but suddenly they land on a Russian submarine during the Cold War. When the submarine sinks, the Tardis vanishes and the Doctor and Clara are stuck on the submarine. As if this wasn't enough, they soon find out that the crew has dug an Ice Warrior out of the ice of the North Pole who now wakes up after sleeping 5000 years. The Ice Warrior tries to contact his people but when he gets no answer he tries to kill everybody on board the submarine and to launch the nuclear missiles. He wants revenge because he thinks all of his people are dead. The Doctor and Clara try everything to avoid a war that would change the future of humanity but it's not so easy especially when you are scared like Clara. When the submarine sinks deeper, a spaceship appears suddenly and brings it to the surface. Will everything turn out alright now?
I liked this episode more than the last one. Maybe it was because it had more action or because it played in a submarine. In this episode the monster was more present than in the last one because an Ice Warrior is something different than a being that looks more like a sun. It was really creepy when the warrior got out of it's armour because you didn't really see him only his hands or a part of his face. So it's still a mystery how he looks beneath it. The setting of the submarine makes everything a bit more dramatic because you know that there is now way out for the Doctor and Clara with 700 metres water above them and the Tardis gone because of an emergency protocol. This episode was darker than the ones before but it still had some funny moments in it. I really liked the mix of dark and light moments that Mark Gatiss has achieved in this episode. This was probably the first episode in which Clara was really scared because until now she had only seen the exciting but not that dangerous things with the Doctor. Now she knows that there will be situations where even the Doctor seems to be helpless sometimes.