Ash loves two things: his family and catching criminals. A botched operation leads him to join the Unit, a new undercover law enforcement team, operating outside of the police but with one goal in mind - to get to the criminals at the very top, the ones most skilled at evading justice. Catching the biggest criminals is music to Ash's ears. Here is his chance to make a real difference, to put the most ruthless criminals away, the ones who do the most damage in society and mostly get away with it. But his other more secret agenda in joining the Unit is revenge. Ignoring his wife Lorna's fears of the risks this new job might entail and her dreams of a safe life for their family, he throws himself obsessively into his work. Driven by the ghosts of his past, and uncompromising in his approach, Ash's knowledge of the street soon starts to pay off. The Unit get under the skin of a serious criminal drug gang, but as they begin to climb further up the organisation's hierarchy and identify the key members, Ash's two goals in joining the Unit are brought into sharp conflict. When it comes to it, can Ash put his personal feelings aside in order to learn more about an organisation bigger than any of them could have imagined? And if he can't, where does this leave his role in the Unit?

Ash is closer to the bigger criminals than he has ever been. The Unit has its hooks into an important drug dealer: Xavier. They're following his every move in the hope he'll lead them to someone even higher in the organisation, and ultimately to the man at the very top. They follow Xavier to a meet with a rival dealer in a busy market. Events take a shocking turn when guns are drawn, panic ensues and an innocent boy is killed.

The Unit are beginning to make inroads into the criminal organisation. Ash's job isn't great for family life, but he's doing the kind of work he's dreamed of. Phone taps have just revealed that a big sum of money is being sent out from Xav's dry-cleaners on behalf of 'someone bigger', and it's a great lead - if they follow the money, it could lead them back to its source, and one step closer to identifying their 'Mr Big'. Tracking it reveals the existence of a money launderer known as the Accountant, who moves dirty money in and out of the UK using various ingenious methods.

The Unit's war of attrition on Roach and his gang is beginning to take effect - under pressure, and low on drugs and money, they're forced to turn to a new foreign supplier, a Norwegian nicknamed the Viking. The Unit know how vital the Viking could be in learning more about the crew they're targeting, but he's proving frustratingly hard to pin down. Clever and terrifyingly ruthless, the Viking thwarts the Unit at every turn, and the team find themselves at odds with each other about the best way to proceed. If they fail this time, the city will be flooded with a tonne of dangerous drugs, and Roach will be a step closer to achieving his ambitious expansion plans.

A young girl is dead - the second pill-related death in recent days. A new batch of toxic pills has hit the market, and the police are struggling to find the source. When the Unit discover a link between the pills and the gang they're investigating, Cartwright agrees it's worth pursuing, but only if it's to further their aims of getting higher up the organisation. No-one wants this more than Ash, but he's equally determined to stop more lethal pills from hitting the streets. As Roach waits to hear news of his foray into the pill business, he's unaware of the net closing around him.

After the failure of the pill operation, Roach turns to Yorkie, a violent armed robber, to solve his cash flow problems. Troubled by the terrifying intel they gather on Yorkie, and his links to their crew, the Unit set out to find and track him. Yorkie could help them get deeper, but with Cartwright's old adversary DCI Stannard determined to shut down the Unit, finding a wanted man like Yorkie could also get the police off their backs.

Ash returns to work following his violent encounter with Yorkie and becomes convinced a big shipment of drugs is being planned. Cartwright is sceptical, so Ash secretly begins to investigate on his own. Kim is horrified to discover that ex-boyfriend Connor has used their relationship to feed intelligence to Stannard, while extensive surveillance on Docker yields nothing, but the discovery of a new player keen to muscle in on Roach's organisation could lead them to finally putting a face to the figure at the top of their board.

With Roach's identity finally revealed, and the villain seemingly within the Unit's grasp, the team now face the challenge of intercepting his massive shipment of drugs and, even more crucially, somehow linking it to him. Meanwhile, Roach is feeling under pressure from both outside and inside his organisation. With Casby feeding his paranoia, Roach becomes convinced that another crew is after his business, and that there is a traitor within his own gang. But what is he prepared to do to restore order in the ranks? As the Unit close in on what they believe to be Roach's shipment, little do they know that Roach has a final surprise up his sleeve. Will Ash finally manage to bring down his biggest ever criminal adversary, and if he does, what will it cost him morally and personally? If, after it is all over, he can't get his family back, will it all have been worth it?