Parts: 1 (45-minute)
Writer: Mark Gatiss
Airdate: 13 April 2013
Production Code: n/a

Doctor: eleventh
Companions: Clara

Story: Expecting to step out of the TARDIS and into glitzy Las Vegas, the Doctor and Clara are surprised when that first step lands them in the control room of a flooded submarine, as frigid Arctic water bursts through the hull. Having temporarily stopped the sub from sinking deeper, thus saving it from being crushed under the immense water pressure of the inky depths, the Doctor soon learns that the sub's troubles began when its Russian crew brought on board a presumed mammoth that they found frozen in the ice. One of the crewmembers unwisely freed the "mammoth" from its imprisonment, unleashing it upon the vulnerable submarine... and when it marches up to the bridge, the Doctor immediately recognizes it as an Ice Warrior.

Not just any Ice Warrior, either, but a long-vanished and famed warrior named Skaldak. Knowing him by repute, the Doctor is immediately concerned with persuading the dangerous Skaldak to ally himself with the human crew... but any such hopes are dashed when one of the Russians attacks, causing Skaldak to declare war on the entire submarine, along with the planet Earth itself. With the submarine slipping closer every minute to sinking deeper beneath the ice, and Skaldak prowling the darkened drenched corridors with the double aim of slaughtering the crew and provoking a planetwide nuclear war in 1983, the Doctor may face an insurmountable task in attempting to broker a peace in which everyone survives...