The 18 certificate awarded to Matthew Heineman’s documentary about the citizen journalists behind the website Raqqa Is Being Slaughtered Silently gives some indication of the horrors that these brave men and women witness on a daily basis. Political activism was not even on the radar of the inhabitants of the isolated Syrian city of Raqqa until the Arab spring ousted the government forces and the resultant power vacuum was filled by Islamic State. Watching with mounting dread the atrocities meted out by the men who claimed to be the city’s liberators, a loose group of activists vowed to bring the truth about life under Isis to the rest of the world. This documentary doesn’t spare the audience from the obscenity of the violence – this is a gruelling but essential watch. We see executions; we see bodies decapitated and displayed as a stark warning to the people of Raqqa and the world beyond.

However, ultimately it’s not the violence that is most disturbing. It’s the effect on the central characters of the film, forced into exile, tallying each new death, each tortured friend or family member from afar. There’s one chilling scene when spokesman Abdalaziz Alhamza flicks through a treasured cache of photographs of loved ones and starts to shake, uncontrollably, his body shaking with traumatic stress. The ghosts of his city haunt him.