There were two things I wasn't prepared for in this movie. One was the very ambiguous ending, and the other was what I think of as the political aesthetic. Friedkin obviously did a huge amount of research and despite what looks like caricatures this is probably how gay men still in the closet dressed and acted in the gay bit city American underground in late 1970s New York.

I can also see how real gays at the time were outraged. Friedkin seemed to be committed to showing the subculture in all its raw, angry sexuality with the hair, leather, costumes and aggressive sexual predilections, but the premise itself seems to be a Middle American Christian fantasy (one they'd feel vindicated in a few years later with the alarm surrounding the spread of HIV) – being gay is dangerous and might get you killed.

It's framed as a simple undercover detective story as young, clean cop Steve (Pacino) is sent into the undercurrent of the lifestyle to flush out a serial killer targeting gay men. His no nonsense Captain (Sorvino) picks him because he has a passing resemblance to the guys the murder's topped, and Steve has to adopt the look and lifestyle, going to the dingy leather bars and pretending to be available and pick up guys, all the while making connections and asking around for what he can find out.

But it's more subtly about identity and how Steve feels both the thrill and the (apparent) moral rot of this new world becoming part of him, and he a part of it. The final shocking killing – coming after the cops apparently have the murderer they've been looking for – and the final inconclusive frames zeroing in on Steve's knowing and suspicious face make us wonder if he got into the gay lifestyle too much, hated himself for liking it, and struck out.

In one interpretation it actually makes it even more a straight Christian fantasy too – asking (but not answering) the question of whether you can choose or 'catch' being gay from the squalor of sleazy bars and random, meaningless sex with other men.

The trappings, script and design are very much of the era. The killer's low, distinctive voice a bit too obvious as a marker for who he is, but it's the only real stylistic misstep.