In 2014, director Jamie Roberts filmed with Islamic extremist Abu Rumaysah. This month, Rumaysah has become one of the world's most wanted men: he's suspected of being the British jihadi in the latest Isis execution video. This timely, powerful new hour-long documentary for Channel 4 takes an intimate look at the people spreading extremist fundamentalism in Britain. Roberts - who also directed Angry, White and Proud for Channel 4 - gains extraordinary access, over the course of two years, to a new wave of extremists, including Rumaysah, who are radicalising and grooming young British Muslims.