They say that the first two weeks of Royal Marine training are among the toughest of the whole 32 weeks. This first episode follows 56 new recruits through this so-called 'shock of capture'. The recruits spend these opening weeks living in a 60-bed dorm where their every waking hour is controlled by their drill instructor, Corporal 'Froggy' Chauffour. Froggy is a Frenchman with a passion for discipline and high standards, all delivered with his trademark toughness and a glint of humour in his eye. Like rabbits in headlights, the recruits struggle to cope with the gruelling demands of their new way of life, toiling tirelessly to master the details, stay awake and even suppress giggles in the face of Froggy's wrath. Some question their decision to join up, while others have their career as a Marine cut short when it's barely begun. By the end they must all pass a final inspection on everything they've learned so far, which for this troop in particular looks like a tall order.

The raw recruits take the next step on their training and there are challenging new lessons at every turn. If they can make it through the next three weeks, their families will get invited down to witness their progress and they'll get to go home for the first time since training began. As the training intensifies, 18-year-old Callum Bond tries to face up to a bewildering array of challenges in an effort to transform himself from boy to soldier. At the other end of the age spectrum, 31-year-old Magnus Frykman struggles to remain on top of training while missing his family, leaving his chance of a weekend at home in the balance. Out on exercise, running through their first rifle drills, the recruits begin to come to terms with what they may be required to do if they make it through training and get their green berets. And, as if things couldn't get any tougher, a familiar face, Corporal 'Froggy' Chauffour, is called back in to get them up to scratch. 'Definitely the worst I've had,' he says, 'they need a bit of a slap on the wrist, get back to work.'

Six weeks into their training, the marine recruits face their first pass-or-fail test. They must complete a tough series of physical challenges in the gym, culminating in three staggeringly high rope climbs. If they fail to complete it they'll have to leave their troop. Dorm-mates Borg and Marks are each up against it, for very different reasons. Borg, who's 22, has arrived from Malta to pursue his childhood dream of earning a green beret. He's not built for scaling ropes and he prepares by gorging himself on jelly and tins of fish. He's hanging on to his place in the troop by a thread. Thirty-one-year-old Marks from Portsmouth is hoping to put a difficult past behind him by training as a marine. His quick wit provides endless entertainment for his fellow recruits but the trainers often interpret it as an attitude problem. The instructors worry that neither of them may be cut out for military life; can the recruits prove them wrong?

The troop 174 recruits reach the challenging halfway point in training. The second half of the course includes more challenging exercises to prepare the recruits for frontline fighting, but this is traditionally a point when many recruits leave the troop, either through failure or giving up. Dilliway, whose father was in the navy, has wanted to be a marine since he was a boy. As training steps up a gear he faces tough questions and events take an unexpected turn as the troop hits 'crash week': the preparation for one of Lympstone's most notorious physical tests, on the camp's iconic Bottom Field Assault Course.

As training progresses, exercises start to imitate life at war, and the nature of the job begins to worry some of the recruits of Troop 174. University graduate recruit Greene's initially impressive performances are rewarded with a section leadership role, but he is yet to tackle one of training's biggest challenges: the Bottom Field assault course - a set of seemingly-impossible obstacles, tackled while carrying heavy loads of kit. As the Bottom Field test moves closer, problems pile up and Greene's chances of passing look remote. Added to this the troop has a new boss, Captain Simon Smith, who is under pressure to guide Greene and the rest of his troop through this challenging time with as many of them as possible still on track for the Green Beret.

As training progresses, more and more recruits start to struggle with the tough demands placed upon them. Cole has been at Lympstone for three months but has had to spend most of this time in rehab with a knee injury. Now he returns to mainstream training and attempts to make it through a five-day exercise on Dartmoor. Meanwhile, the mental demands of the job are causing Seda from Egypt strife. He decided to join the Royal Marines after reading an article about the training in Men's Health magazine, but the realities of the job are weighing heavy on his mind. As the lessons about being trained to kill and how to engage in hand-to-hand combat pile up, he faces a tough decision about whether he should be a Marine after all.

The recruits are within touching distance of their green berets, but first they must face Lympstone's ultimate challenge - the famous commando tests - four gruelling physical exercises attempted over five consecutive days. The members of 169 troop are a tight-knit group who have pushed through 30 weeks of training together. They're desperate to finish the course as one, but not everybody will make it

As the recruits reach the end of training, thoughts turn to life beyond Lympstone. A familiar face, Corporal `Froggy' Chauffour, returns to prepare the troop for a final pass-out parade, and four contenders compete for the coveted King's Badge award, which the winner will carry with them throughout their career. Meanwhile, retiring Troop Commander `Ginge' Booth contemplates a future beyond the Royal Marines after 28 years of service.