Ex-police officer Rachel runs her first undercover operation as Field Office 19 investigates the activities of a couple of suspected arms dealers. When Field Office Leader Hannah is notified of a Prime Ministerial visit, she suspects an assassination plot, but will the team find the hitman in time?

Hackers use the state Emergency Communications System to broadcast what they claim to be video footage of dead prison inmates. When MI5 denounces the broadcast as a hoax, terrorists set off a series of false alerts. Mathematician Charlie suspects MI5 activity is related to the London bombing.

Jez and Kylie go under cover on a notorious housing estate to investigate a scam involving the sale of counterfeit radiation sickness medication. Police officers question Rob about the death of a fellow medical student at the time of the London bomb. Rachel and Charlie try to uncover the identity of the traitor within MI5.

The horrific murder of two MI5 informants lead the team to discover a leak at a government-run Missing Person's Scheme. The race is on to find the source of the corruption and stop the killings before they escalate. Rachel goes undercover to find further information on the MI5 cover up and learns a British scientist may have been wrongly imprisoned for his part in the bomb plot.

A bank heist by eco terrorists, involving a 16-year-old suicide bomber, leads the team to uncover a secret deal between the British government and a Korean billionaire. Will the team be able to free the hostages and stop the terrorists from completing their deadly mission?

Charlie is convinced that Abid Malik can lead him to the MI5 traitor and prevent a second bomb attack. Vik and Rob are sent under cover to Camp Windsmere to break him out.