A serial killer (Mehdi Bajestani) stalks the Iranian city Mashhad, strangling sex workers and dumping their bodies near the outskirts of town. With the local police seemingly unable to track the murderer down, a bold journalist from Tehran (Zar Amir Ebrahimi) goes to extreme lengths to identify and apprehend him.

The most disturbing part of Ali Abassi’s Holy Spider is not the grisly subject matter, nor the blunt and unpleasant depictions of several murders, but the mere fact that it is all based on a true story. Details have been changed – Abassi’s dogged protagonist Arezoo Rahami is entirely fictional – but the factual elements remain alongside them. It is a matter of public record that between 2000 and 2001 Mashhad labourer Saeed Hanaei abducted and murdered 16 women. Once arrested and put on trial, some religious conservatives not only pled for leniency but campaigned for his release.

It is a difficult subject matter, and a confrontational film. Abassi was refused permission to film in Iran, with the bulk of production undertaken in Jordan with a German-led pan-European budget funding it. His screenplay (with Afshin Kamran Bahrami) comes straight from the playbook of a Thomas Harris potboiler: a narrative split neatly in two, one following the killer and the other the journalist investigating him. Much of the suspense comes from waiting to see these two threads overlap and interact. By introducing a fictional female journalist in the latter thread, Abassi affords himself the opportunity to tackle the struggle of women in Iranian society head-on. Arezoo faces enormous challenges in her job, not only everyday sexism but stonewalling her investigation, deeply threatening sexual harassment, and physical violence. The oily sense of misogyny is ever-present: oppressive, unpleasant, and never-ending.

The success of the film hinges on its lead performances. Zar Amir Ebrahimi brings a powerful drive to Arezoo, and a deep complexity that underlines each line, look, and pause throughout. It’s little wonder that her acting here won her Best Actress at Cannes. It is magnetic. Mehdi Bajestani performs what is effectively its mirror. In his hands, Saeed is a more complex and multi-faceted character than viewers may be comfortable watching. We see him as a religious zealot, a loving father, a spiteful murderer, and a somewhat pathetic coward. He is both emotionally broken and forcibly driven. In short, he feels real: never are such reprehensible actions so difficult as when they are presented with such honesty.

There seems to be an never-ending stream of film and television dramas about serial killers, even as the real-life incidence of such criminals is in an ongoing decline. Audiences enjoy wallowing in such bleak misery, it seems, vicariously experiencing shock and dread as they go. The realism and social context of Holy Spider make that process a little more difficult than they might expect. By its conclusion, it is actively – and provocatively – nauseating. Some times the most shocking of narrative films can be created by simply showing the world as it is.