There's a bit of hype surrounding this episode as it sees the return of the Ice Warriers, one of writer Mark Gatiss' favourite Doctor Who villains. I'll be honest, I haven't seen them in action, but I was excited to see the submarine regardless. The pressure underwater is ramped up, so throw in a manic Doctor and who knows what will happen! The official BBC blurb says: "On a Russian submarine in 1983, a frozen alien warrior is waking up, just as the TARDIS materialises." Excellent timing, Doctor.