Filmmaker Robb Leech attempts to understand his stepbrother's journey from middle-class white boy in Weymouth to convicted terrorist. In 2010 Robb spent a year filming his stepbrother Rich after he turned his back on the world in which he grew up to become a fundamentalist Muslim called Salahuddin. Robb began filming with his stepbrother as he entered a strange new world where everyone talked about fighting jihad and implementing Sharia law. The result was Robb's acclaimed BBC Three documentary, My Brother the Islamist. When, in 2013, Salahuddin is convicted of preparing terrorism acts and jailed for six years, Robb is desperate to know what triggered his stepbrother, and others like him, to cross the line. Robb seeks out imam and psychologist Alyas Karmani to understand what drives young British-born men and women into radical jihadism. And he confronts Anjem Choudary, the man who converted Rich, about his role in Salahuddin's radicalisation

• Documentary