With a mass-casualty terror attack in the UK seen as almost inevitable, this drama documentary dramatises what terrorism experts fear is the most likely scenario for Britain's next major terror attack. Counterterrorism police believe that the greatest threat to our security comes from Marauding Terrorist Firearms Attacks (MTFA), which can result in dozens of fatalities even if armed police respond within minutes. There are more than a thousand high-priority terrorism suspects in the UK, but there are only enough surveillance officers to monitor a fraction of these at any time.