Watch How to Command a Nuclear Submarine

It's the final countdown as the remaining students must face their toughest tests yet in their bids to pass the Perisher course and become qualified to command their own nuclear submarines.

Three of the four remaining students face a new scenario as they take part in one of NATO's largest naval exercises, "Join Warrior," off the west coast of Scotland. Working under orders from flagship Ark Royal, the students must command HMS Triumph as she takes on deadly opponents in a series of sub-versus-sub duels.

The five students take on a new challenge that includes crucial aspects of modern submarine operations: they'll be sneaking around in dangerous shallow waters, conducting reconnaissance missions and dropping sensors across the mouths of harbors.

We meet five young officers who participate in the Perisher Course as they begin the all-important sea phase of the program. Four come from the Royal Navy and one American is from the US Navy, which regularly sends one of their top officers on the course.