Only five years into his life sentence for killing his aunt and uncle, a crime committed when he discovered their abuse of his sister, John Mercer is released from prison. He’s introduced to Lenny Douglas, a police officer “retired on grounds of ill health.” In fact, Lenny is an operative whose remit is to take down those members of the criminal and official establishment who think they are above the law. His task is to bring order where the law has failed. Mercer’s freedom comes at a price. He must use the skills he gained serving in the Special Forces to remove untouchable criminal Jacob Gould who was responsible for a hit on a senior police officer. Mercer is put in a south London council flat with Calum MacKenzie. Calum is an old cellmate and annoys the hell out of Mercer. He is an irrepressible bundle of deviant energy whose main interests are girls, drugs and ‘slamming tunes.’ But, to Lenny, Calum is an invaluable snitch and expert breaking-and-entering man.

Following the death of the head of a notorious crime family, Lenny visits his successor, the police mole Jude Cassidy. Lenny wants to persuade him to come under police protection. Jude and an accomplice have recently been acquitted of the murder of a young black man whose cousin, Elviss Gilroy, is making plans to avenge his death. Reluctantly, Jude agrees to go into hiding, under the protection of an even more reluctant John Mercer. Leaving Calum to watch over Jude, Mercer goes to visit his sister Jess. Cooped up in the little council flat, it takes no time for Jude’s true nature to emerge and he starts taunting Calum about his mother. Mercer gets a call from a hysterical Calum, who has lashed out at Jude with terrible consequences.

Lenny is alerted to the recent arrival of a notorious Albanian gangster Tarek Sokoli. Sokoli wants to put down roots in his new homeland and that includes running drugs and prostitution rackets. Rather than have him killed, Lenny instructs Mercer, Rose and Calum to scare him away - to send the message to any others planning on setting up in London that their attempts will fail. He wants them to work as a team. Calum frees the prostitutes that Sokoli’s crew had illegally shipped in by returning their passports. However, after one of the liberated girls is killed, and a heroin importer is murdered by Sokoli’s lieutenant after Mercer convinced him against doing business with the Albanians, the unit begin to realise that simply executing Sokoli would have created far less bloodshed.

Calum has made Mercer some tea. Problem is that it’s burned and totally lacking in nutritonal value. So off they go to the supermarket. Calum is amazed Mercer knows so much about cooking. Lenny appears and Mercer's heart sinks. He gives Mercer his next assignment. He must kill a violent criminal just acquitted of the murder of a young woman, Marie Greene. His name is Scott Glover. Mercer finds him at a hotel and prepares to kill him in the gentleman’s toilets. But his plan is fraught with problems. One of which is that a dogged and brave reporter knows more than he should about the case.

John Mercer’s predecessor, Patrick Finch, has gone on a vigilante-style rampage, meting out his particular brand of justice against drug-dealers and prostitutes. He is working on his own and he’s making newspaper headlines. It’s time to put the brakes on Finch. Meanwhile, Hugh Berry, the man who sorts out the paperwork to cover Lenny’s illicit activities, asks him to take out a hit on his behalf. A hit-and-run driver Trevor Bowyer rendered Hugh’s grandson mentally disabled. But the driver only served two years for the crime. Lenny is left in no doubt that if he doesn’t send his hit man to kill Bowyer, Hugh will find one of his own. Lenny visits Finch in an attempt to bring him into line. But he finds a bitter, alienated and menacing man. Finch is everything Lenny feared he might become. Leaving Finch’s tower block, Lenny is furious to find Hugh Berry waiting outside, he’s clearly tempted to hire Finch in order to take out Bowyer.

Lenny is delicately closing the net on the powerful jury-fixer, Richard Blakeney. But when Mercer discovers just how deeply Rose is involved in the operation, and how much Lenny has been holding back, he vows revenge. As Blakeney tempts the disaffected Mercer into working for him - and getting rid of Lenny - Mercer is left to make a decision that could cost the lives of all he holds dear.

The first of a two-part story, the second season opener finds the team battling a ruthless street gang that traffics in more than just drugs. Lenny is concerned about the future of his underground crime-fighting crew and faces a threat to his authority from within MI6.

The team are assigned to stop a drug dealer whose Afghan operation has cost the lives of some British servicemen there. The battle becomes very personal for Mercer who must overcome an injury in his determination to bring the man down.

Rose is stalked by the man who got her thrown off the police force. Innocent bystanders may end up drawn into a cat and mouse scenario as the crew attempts to deal with a rapist & murderer before he escape justice again.

The team find themselves involved in the world of cage fighting when they try and deal with a gang plotting a major heist.

A hitman from India arrives in the UK with the intend to kill a ten month year old baby who is the heir to a large fortune. Mercer and the team have to work to protect the child and stop a mob war from breaking out. Symmonds attempts to destroy the team which leads to repercussions.