Set 21 years ago in Masshad, Iran, Ali Abassi’s Holy Spider is a disturbing (to put it mildly), fact-based drama about Saeed Hanaei (Mehdi Bajestani), a serial killer of prostitutes.

The murders are ghastly enough, but a double-down comes when, post-capture, Hanaei is bizarrely supported by fanatical zealots who believe he has done Allah’s bidding.

The first half is pretty much a straightforward crime drama. After graphically depicting two of Hanaei’s grisly killings, it follows an intrepid female reporter (Zar Amir-Ebrahimi) who risks life and limb to bring about his arrest.

I can’t call this section any more than decent — efficient and good enough, but not exactly brimming with style or suspense or cinematic flair. Okay, the suspense does ratchet up when Amir-Ebrahimi’s journalist character Rahimi (who’s actually fictional), posing as a prostitute, is being driven by Hanaei to his home, and Amir-Ebrahimi’s colleague does a poor job of following them and suddenly you’re thinking, “oh, God…she could be next.”

The diseased social reaction among Hanaei’s fans in the second half is what shakes you. You’re left thinking “really?…a sizable contingent of Mashhad citizens cheered a serial killer because he was helping to rid the streets of streetcorner hookers? Who thinks like that? What kind of diseased culture?,” etc.

But then of course, this was Iran 20 years ago (the murders happened between ’00 and ’01) and the Masshad faithful were the country’s chief bumblefucks. All you can say is “wow.”