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.