Heroes emerge in the shadow of the so-called Islamic State, and those heroes get their moment in Matthew Heineman's absorbing documentary "City of Ghosts."

The movie, which debuted at this year's Sundance Film Festival, follows the men behind the citizen-journalism group Raqqa Is Being Slaughtered Silently, which chronicles the oppression inflicted on the Syrian city of Raqqa by the group known as ISIS. Their mission is to present a counternarrative to ISIS' slickly produced propaganda, which argues that the people of Raqqa love their new rulers — in spite of the regular executions and beheadings.

These journalists work anonymously, for the simple reason that getting caught will get them killed. This goes both for those still in Raqqa and those who have escaped to Turkey and Germany, where some have been assassinated by ISIS followers abroad.

The journalists inside Raqqa shoot the footage and download it to their exiled colleagues, who then post it online for the world to see. (The movie shows many examples of mainstream news outlets, such as CNN, using RBSS' footage.)

Heineman, who followed Mexican vigilantes fighting the drug lords in "Cartel Land," once again risks life and limb to tell a compelling story. He gets interviews in Syria, where the threat from ISIS is ever-present. He also profiles the exiled RBSS journalists, who fear retribution not only from ISIS but also from a growing white nationalist movement in Germany that sees Syrian refugees not as victims but as a menace.

As with "Cartel Land," Heineman lets his subjects carry the story in "City of Ghosts," as they recount friends and family who have been killed by ISIS — and how that loss spurs them to continue with their vital, dangerous work.

'City of Ghosts'

Journalists turn activists to get the real story out of Syria in this absorbing documentary.

Where • Broadway Centre Cinemas.

When • Opens Friday, July 28.

Rating • R for disturbing violent content and some language.

Running time • 92 minutes.