The first episode sees the recruits arrive at an isolated barracks in Wales, and being introduced to the directing staff of Foxy, Ollie and Colin, all under the watchful eye of chief instructor Ant. The recruits' first task is an unexpected one: writing an essay about themselves. Then they face an eight-mile combat fitness test while their essays are dissected for an insight into their character.

Following a brutal first stage, only 20 of the original 30 civilians remain and the directing staff ratchet the intensity up a notch, applying pressure to isolate and expose individuals who they think are not up to it. The remaining recruits must choose their weakest colleagues, who then lead the next exercise. And there's the 'Sickener': a series of physical challenges that won't end until five recruits give up.

Chief instructor Ant and his team play mind games on the 12 remaining recruits to see if they have the mental strength to keep going, or if anyone will voluntarily withdraw from the process. They speak to each recruit individually, giving them negative feedback on their performance so far and questioning whether they have what it takes to make it to the end. Those who can keep their resolve face one of the most intimidating tasks on Special Forces selection, diving backwards off a cliff into the freezing sea 12 metres below.

The directing staff have to whittle the group down to six from 11 and have a surprising trick up their sleeve that they think will cause some participants to instantly withdraw before the final stage. Those who make it through the cull are faced with their biggest challenge so far - an escape and evasion mission designed to prepare recruits for avoiding capture behind enemy lines. They are divided into two teams and dropped off at dusk, having to cover miles of rough terrain overnight in order to hit their first rendezvous point in nine hours.

The remaining recruits each face the final, notoriously gruelling stage of selection alone: 24 hours of intense interrogation without losing composure or revealing details of their mission. Between interrogations, they are locked in a cell and hooded in stress positions and are subjected to repetitive music at random intervals to maintain a state of severe fatigue. Less than 10 per cent of candidates make it through this process in the real world. Will any of the civilians make it to the end and prove they have the characteristics needed to succeed in the special forces?