When Scarlet bursts in on Captain Black buying technology from Russians, a chase develops that leaves them both trapped in an immobilised Rhino vehicle at the bottom of a frozen lake. The only chance of escape is to work together and Scarlet uses the opportunity to try to reach the man inside the Mysteron agent, the man who was his friend.

Two mortal enemies having to combine forces to stay alive is hardly an original concept, but here it is well used to play with the mythology of the show. The interplay between Scarlet and Black is as close as the show has ever come to emotional complexity and making Black more human than his name suggests is an interesting move.

The lake beneath the frozen surface appears strangely enough to be almost tropical with seals, shoals of fish and coral outcroppings that put it at odds with the impressively icy world above.

Interesting questions are raised about the nature of Mysteron agents as well. Scarlet is indestructible, but what happens when there's no air? Black claims to have the memories of the man he was and that perhaps his personality is still there, just being suppressed by the Mysteron consciousness.

Of course, you could just enjoy it as an adventure yarn, which is exactly what it is filmed like.