Multi-tasking film-maker Matthew Heineman embeds himself among Mexican drug cartel members and, in more detail, the vigilantes pushing back against them, but the closer he gets the harder it becomes to tell the difference between the two theoretically opposing camps.

Two main protagonists emerge: Dr Jose Manuel Mireles, the charismatic leader of Autodefensa, a ragtag army trying to drive the cartels out of rural towns in the state of Michoacan, and Tim Foley, an equally watchable if more obviously sketchy (at least at first) militiaman who leads vigilante group Arizona Border Recon. Survivors testify about unspeakable atrocities, corruption is revealed, and the misery keeps on going in a perpetual spin-rinse-repeat cycle of horror.

There’s no doubting Heineman and his crew’s audacity as they venture close to the line of fire, but the commitment to observing dispassionately at all times starts to feel a bit like a cop-out.