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 & 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
- & 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 & follow their trail. He enters a dark world of human trafficking befriending a vicious criminal ring of sex traders & putting himself at their disposal. As Murphy hunts for the missing officers he uncovers evidence suggesting they may have disobeyed orders & gone too deep undercover. Can Murphy save himself & the missing officers or is he in danger of becoming the very thing he is out to destroy?