:Episode 3 - Natural Selection:

Two hundred hopeful recruits to the SAS have been reduced to 10, but for these men the toughest test is still to come - a three-day combat survival exercise in the Welsh mountains, under the watchful eye of Henno and Red Troop.

Dressed in World War Two battledress - rough trousers without belts, greatcoats without buttons and boots without laces, they must navigate 100 miles across rough terrain. To help them, they are given a home-made silk map, two 10p pieces, matches and a compass. And to succeed, they must evade capture by 40 Paras acting the part of the enemy in the woods.

Mick Sharp hopes to join his mate Jamie in the regiment - it's his last chance to pass selection as students are given two strikes then they're out. Henno is out to make life difficult for him and puts him in a team with an injured soldier, Luke, plus two others Trent and Louis. In another part of the woods, meanwhile, a man arrives, dressed in camouflage gear.

Stumbling at night, Luke is separated from the others and is smashed in the face with a tree branch. His attacker tells him he'll never make the grade. The remaining students are swept away as they struggle to cross a river. Mick and Louis haul themselves out and go in search of Trent. They find him dead.

Mick reports the suspected drowning, but when Trent is airlifted away doctors discover he was murdered. Against police orders, Red Troop go to hunt down the killer. The ordeal is still not over for Mick...