Andrew Buchan plays John Mercer, an ex-soldier serving life for murdering his aunt and uncle after they molested his sister. One day, ex-cop Lenny Douglas visits him and offers him a release from prison...
So he does, aided by small-time crook Calum — an annoying but loveable sidekick — and Rose, a former policewoman and Lenny's ex-wife.
This show contains examples of:
- And Your Little Dog, Too!: Mercer refuses to live with Calum as he found him annoying as a cellmate, so Lenny has Calum garroted until Mercer relents.
- Anti-Hero: Calum's a Type I occasionally wandering into Type II, Mercer and Rose are Type IIIs, and Lenny's a Type IV frequently straddling Type V.
- Black-and-Gray Morality: Mercer's hits are about protecting the Establishment and keeping British society stable, rather than punishing evildoers.
- Bumbling Sidekick: Calum
- Cliffhanger: The ending of the second season.
- Good Is Not Nice: Hot-tempered, coldly regimental Lenny all the way. Mercer, too, for the most part, though he has more Pet the Dog moments, mostly with Calum and his sister.
- Honey Trap: Rose Chamberlain was thrown out of the police for trying this tactic to snare a rapist, which was deemed "entrapment". She continues to do so however to gather information for the unit, much to Mercer's distaste.
- Latin Lover: Manuela
- Morality Pet: Mercer's sister.
- Plucky Comic Relief: Calum
- Professional Killer
- Rogue Agent: In one episode Mercer is ordered to kill his predecessor, who has started to pick his own targets instead of those selected by Lenny. On another occasion an SAS man who (under orders) used drug money to finance his anti-Taliban operation decided to go into business for himself.
- UST: Between Rose and Mercer. Calum often lampshades this.