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’. Mercer soon discovers that his newfound freedom comes at a price.

With the head of a notorious crime family dead, Lenny visits his successor, the vital police mole Jude Cassidy, to persuade him to come under police protection. Jude and an accomplice have recently been acquitted for the savage 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 who is forced to take him in.

Lenny is alerted to the recent arrival of a notorious Albanian gangster Tarek Sokoli. Rather than have him killed, he 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. The unit target Sokoli's two main sources of income – prostitution and drugs.

Fed up with their lifestyle, Mercer takes Calum on an educational trip to the supermarket. Lenny finds them there and orders Mercer to kill Scott Glover, a man just acquitted of the murder of the young, innocent, 20-year-old Marie Greene. Mercer questions why they are killing a man innocent in the eyes of the law. Lenny's reluctance to divulge any further information has Mercer's interest piqued.

Rose tells Lenny that a former member of Lenny's unit – John Mercer's predecessor, Patrick Finch – has gone on a vigilante-style rampage, meting out his particular brand of justice against drug-dealers, as well as two escort girls. Lenny tells Mercer to kill Finch, but events soon get complicated. Meanwhile, Hugh Berry, the man who sorts out the paperwork to cover the unit's illicit activities, asks Lenny to take out a hit on his behalf. A hit-and-run driver Trevor Bowyer rendered Hugh's grandson, Owen, mentally disabled, but only served two years for the crime. Whilst emphatic that they are not a bunch of vigilantes, Lenny is left in no doubt that if he doesn't send his hit man to kill Trevor Bowyer, then Hugh will find one of his own.

Lenny is targeting the kingpin of judicial corruption, Richard Blakeney, to expose his network of bent policemen and corrupt judges. Blakeney is everything Lenny hates, and his network is everything he wants to bring down. Lenny briefs Mercer on his delicate strategy, which involves exploiting Blakeney's hit man Georgie, who Calum and Mercer are spying on.

The team are sent to stop a gang terrorising the streets of south London but uncover a child trafficking operation.

With the operation to stop further child trafficking ever more difficult, Mercer and Calum go undercover as prisoners to watch and investigate a big crime boss who is masterminding the child trafficking operation from within his prison.

The team are assigned to take out an ex-SAS trooper turned Heroin smuggler, after broadcasts of the execution of three British soldiers.

Rose's past comes back to haunt her when a target she failed to take down, comes back for revenge, and John is falsely arrested for rape.

Rose and John go undercover in an illegal cagefighting scheme when Lenny orders them to execute a veteran armed robber with a reputation for changing his name and appearance.

A hitman arrives from India on a mission to kill a baby boy who is the heir to a huge fortune. His death would spark off a bloodbath in the Indian criminal underworld.