A botched operation leads Customs Officer Marcus 'Ash' Ashton to join a new undercover law enforcement team known as UNIT (Undercover Narcotics Investigation Team), which utilises various high-tech methods to catch high-level criminals. Ash's street smarts soon prove vital in tackling a major drug ring.

Tommy is finding the transition to UNIT difficult. The team follows every move made by drug dealer Xavier, hoping he will lead them to someone bigger. But when he meets with a rival dealer, things go south.

UNIT use phone taps to track a large sum of money in hopes that it will lead them to 'Mr Big'. But they also lead the team to a family Ash is determined to protect — despite their illegal activities.

Roach is feeling the pressure and turns to a new supplier known as The Viking. However, UNIT are already monitoring the terrifying and ruthless Viking. An old friend of Ash's puts her life in danger trying to assist him.

UNIT uncovers a link between the gang they have under surveillance and a batch of toxic pills. Phone taps lead Tommy to Paulo, a pizza delivery man being used to courier the dodgy pills for dealer Scooter.

Roach turns to Yorkie, a violent armed robber, to try and solve his cash flow problems. UNIT are disturbed by the info they have on Yorkie but he could prove useful in penetrating the gang and keeping the police off their backs.

Problems at home and at work take their toll on Ash, but he's convinced that 'Mr Big' is up to something. A new player on the scene may lead to a breakthrough while Kim realises her ex-boyfriend has betrayed her.

With Roach feeling pressure from inside and outside of his organisation, he becomes convinced that there's a traitor at work. As he attempts to find them, he unleashes an unpleasant surprise for UNIT, leading to a showdown.