This show focuses on the elite plain-clothes detective unit of the Royal Military Police.

Known as the Special Investigation Branch, the SIB is charged with one of the most demanding of all military jobs: the investigation of serious crimes committed by, or against, members of the British Army.

Sergeant Jo McDonagh (Tamzin Outhwaite) is a tough-talking, dedicated new recruit to the unit, who has just been transferred to the SIB from the Close Protection Unit. Jo is a maverick who has to prove herself and gain acceptance in a man's world that is suspicious and unwelcoming towards outsiders.