Twenty-five men and women, including an ex-SAS operator working as a mole, face an intense start to the latest course, led by Chief Instructor Ant Middleton. It begins with a cold-water shock as the recruits must jump from a moving speedboat into the freezing Atlantic Ocean. They then swim to the shore of a remote Scottish island, where the recruits are based for the duration of the course. One among them is an undercover SAS operator, who left the division just over a year ago, but will their cover be blown?