The Doctor headed to the 80s in the appropriately titled episode, "Cold War," this past weekend. As you might guess from an ice-themed set-up like this, it also featured the return of one of the Time Lord's oldest enemies.
The episode opens in the North Pole, on board a Cold War-era Soviet submarine to be exact. After a brief nuclear scare, we soon learn that they've discovered something mysterious buried in the ice. The crew takes a blowtorch to it, and surprise, surprise, there's something moving inside.
Cue the return of the Ice Warriors! Or, more accurately, one Ice Warrior. These reptilian aliens first appeared in the 1967 serial, "The Ice Warriors," featuring the second Doctor. Unfortunately, only four out of the six episodes in that serial exist today, making their return all the more interesting.
But enough Doctor Who history, the newest episode finds the Doctor and Clara unexpectedly arrive on the aforementioned Soviet sub, right in the middle of the Ice Warrior's rampage. The sub is sinking due to damage sustained from the alien attack, but some quick thinking from the Doctor manages to get it onto land. Just when the situation looks stable though, the TARDIS mysteriously disappears and Clara falls unconscious.
When Clara comes to, we find the Doctor and the Captain arguing, only to be interrupted by the arrival of the Ice Warrior. A Mexican stand-off occurs, which reveals that the alien is Grand Marsall Skaldak. Before we can get any further though, a Soviet crewman stuns the Marshall, and the crew take him captive.
With Skaldak chained up, the Doctor reveals why this was a very bad idea. It turns out that the Grand Marshall is the Ice Warrior's greatest hero. Attacking him could mean some terrible consequences for the human race. With that in mind, Clara gets sent in as a neutral party to try and talk Skaldak down. Unfortunately, it seems the Ice Warrior was a little more clever then we gave him credit for, and has already shed his armor and escaped.
Skaldak makes a grab for the crewmember who electrocuted him, and gets him to reveal the idea of "mutually assure destruction." Seeing an opportunity for vengeance, the Ice Warrior decides to launch the sub's nuclear missile, setting off a chain reaction of violence that will likely end the human race.
The Doctor, Clara, and the Soviet crew race to find Skaldak, and just when they think they have him cornered, it turns out he's re-entered his armor and is impervious to their bullets. The episode gets yet another Mexican stand-off, with the Doctor threatening to blow up the sub if Skaldak launches the missiles. Clara reminds the Ice Warrior of his family back home.
Things take an interesting turn however when an entire ship of Ice Warriors shows up, leaving Skaldak in control of the nuke, but with a way to peacefully end the encounter. After a few tense moments, he decides to show mercy and doesn't launch the missile.
As the Ice Warriors fly back home in their ship, the Doctor admits that he'd been fiddling with the TARDIS's "hostile action displacement system," which is what conveniently caused it to disappear earlier in the episode. Turns out, it's currently sitting cozy on the South Pole.
It sounds like the Doctor and Clara have a bit more adventuring to do before their day is done.