When Doctor Who first aired, eras like the 60's and 70's were 'modern day.' Now they're 'history' and an episode set during the Cold War qualifies as a time travel episode.
But "Cold War" is a title of double meaning, and the second refers to something just as dire as communism and nuclear weapons...
Spoilers to Follow
I have to say, I liked Clara better in this episode than any since "The Snowmen." Like many fans, I appreciated her vulnerability, which JLC showed very well without loosing any moments of awesome. Her expression when coming across the dissected corpse and her fear when she finally sang "Hungry like the Wolf..."
Trapped on a Submarine - this isn't an episode for the claustrophobic, and yet there is plenty of movement in this episode. You don't feel trapped, because every minute is important. No dialogue is thrown away. Although I still want to know why the Doctor carries a Barbie doll around with him. Ah well, we will probably never know. (And no, I don't think the blonde hair is a reference to Rose. Pretty sure she'd be insulted by that.)
Favorite character of the episode? The Professor, hands down. He was adorable. Also, kudos to Gatiss for portraying the majority of the Russians as real people, not just 'bad guys.'
And the Ice Warrior? Well, I've never seen any of their original episodes, but was aware of them as far back as the "Waters of Mars" which in some ways is a companion episode to this one. Anyhow, I found him a compelling, creepy and well-done antagonist.
Loved that Doctor wasn't any less a Warrior than the Captain. Such a fun moment.
Felt really sorry for everyone who had to do the whole episode soaking wet.
Is it just me, or is the sonic screwdriver seeming a little too powerful these days?
I'd like to see an 'under the sea' episode where the Doctor and a companion go around in suits or air bubbles and visit something like the Gungan City in Star Wars (which was awesome, despite the fact that JAR JAR lived there)... but until then, this is a pretty good 'Under the Sea' episode.