John MacAllister is an honest copper. This honesty will probably mean that he will stay at his current rank for the rest of his time in the force, after a well known judge was sent down for three years for child pornography despite the attempts by the Chief Constable to get MacAllister to drop the case. He doesn't care. He likes being at the sharp end, has no desire to drive a desk and is respected by his officers.
Then a hit and run by a youth in a stolen car causes his own daughter to be so badly hurt that they finally have to switch off her life support. When the culprit then gets off with 200 hours community service because of his father's position in the community and his contacts with the police and other bodies of influence, MacAllister changes. His patience gone he attacks the youth's father outside of the court and finally gets shuffled out of the force. This turns out to be to his advantage. Nobody kills his daughter and walks away scott free. Use a killer to catch a killer becomes his watchword.