On a sullen July day, ten years on from the terrorist attacks of 7/7, a confident and optimistic British Prime Minister is attending a charity engagement in the East End of London. Suddenly, a man appears from the crowd and shoots him three times in the chest.

So starts this sensational thriller from best-selling novelist and screenwriter, M J Arlidge (BBC's Prime Suspect, the DI Helen Grace novels).

The Prime Minister is dead. Shot down by unknown forces but presumed to be an extremist terrorist attack. Can MI5's Chief Counter Terrorism Officer, Alex Cartwright (Andrew Scott), uncover the truth behind the assassination?

MI5's Alex Cartwright discovers that whoever ordered the assassination of the Prime Minister has covered their tracks very well indeed. Can he uncover them in time to prevent further atrocities?

MI5's Alex Cartwright and Ellen Townsend (Freema Agyeman), his trusted second-in-command, find themselves in a race against time which leads into the murky heart of Westminster... and beyond.

An assassination that threatens to destabilise Western society, power plays in the corridors of power and international crime syndicates intent on their own sinister agendas. Just another day for MI5's Alex Cartwright and Ellen Townsend.

With each political twist and every nefarious turn of M J Arlidge's action packed thriller, Alex and Ellen's pursuit of the truth takes a step closer to its shocking conclusion.

In this shocking episode, Alex and Ellen think they've finally identified those responsible for the Prime Minister's death.

With deception at every corner and danger in the shadows, MI5's Alex Cartwright and Ellen Townsend seem further from the truth than ever. Who can they trust as they continue to unpick a web of political intrigue that appears to span the world?