21 new recruits embark upon a recreation of the brutal SAS selection process, starting with an ambush, a hijack and a 100-foot abseil, which proves to be the toughest course to date.

The recruits face a forward abseil and an arduous beasting by the DS. There's also an extreme water immersion exercise and a team event in a dark network of disused tunnels.

The recruits must master their emotions under extreme pressure - from a scary backwards dive at night to an illusion jump. There is a further shock as the mole in their midst is revealed.

The recruits work in pairs as they tackle the threat of chemical warfare and execute a fast-paced extraction in near-freezing water. Then, it's time for brutal hand-to-hand combat.

The recruits are on the run across the punishing landscape, pursued by a military-trained hunter force, in a real life game of cat and mouse where only the strongest will make it through.

The remaining recruits face the final stages which consist of an interrogation and a brutal sickener. They must prove that they have both the mental and physical strength to complete the process.