A brutal ambush on a police convoy leaves three officers dead and a protected witness fighting for his life. AC-12 turns its spotlight on Lindsay Denton, the officer who led the convoy but escaped unscathed.

Following the tragedy at the hospital, AC12's Kate Fleming is planted undercover in Lindsay Denton's Missing Persons Unit. But Lindsay in turn gathers compromising evidence against the anti-corruption team.

Lindsay is denied bail and finds herself targeted in prison. Growing increasingly desperate, she tells Kate about a relationship with an executive police officer and her theory of why she is being framed.

Following the unhappy attack she suffered in prison, Lindsay is granted special dispensation to visit her dying mother. DCC Dryden shrugs off allegations about his relationship with Lindsay and possible links to the ambush.

Lindsay fights for her life as troubling details emerge of a vicious conspiracy involving corrupt police officers. With growing evidence of links between Dryden and the ambush, Hastings is under pressure to arrest his senior officer.

Cottan and DC Morton uncover the identity of the Caddy, who's been leading the criminal gang operating within the police. Dryden gets charged. Kate is unsettled by Steve's change of heart but is determined to prove that Lindsay was involved in the ambush.