In Series Four Murphy [James Nesbitt] puts his life on the line to infiltrate an unholy alliance between the Johnstone brothers, notorious ex-paramilitary Protestants, and Mo Khan, the leader of a powerful local Asian crime-gang.

During an elaborate drugs sting Murphy befriends the youngest Johnstone brother, using him as a way into the gang. Their criminal world is a complex, volatile, dangerous place - and Murphy has to push himself right to its centre. But this time, has he pushed himself too far?

Following the mysterious disappearance of two undercover officers, Series Five sees Murphy forced back into the field to find and follow their trail. He enters a dark world of human trafficking, befriending a vicious criminal ring of sex traders and putting himself at their disposal.

As Murphy hunts for the missing officers, he uncovers evidence suggesting they may have disobeyed orders and gone too deep undercover. Can Murphy save himself and the missing officers, or is he in danger of becoming the very thing he is out to destroy?