For about 95% of its runtime, the 1983 slasher flick Sleepaway Camp operates as an oddly bloodless, weirdly sexless, overall kind of annoying ripoff of Friday the 13th. Writer and director Robert Hiltzik mines little tension from his story of a youth summer camp besieged by a killer dumping boiling water on pervy camp cooks and stabbing counselors in the shower, Psycho-style. It’s all very rote and by-the-bloody-books until the film’s final minute, when a last-second twist, wonky special effects, and general what-in-the-fuck-ness combine into one of the most enduringly disturbing images in horror movie history. With the movie turning 35-years-old this week, I made a return trip to Sleepaway Camp to discover that yes, it’s still kind of terrible, and yes, that ending will still mess your entire day up something fierce.

First, the set-up: The film opens on a father and two children—a boy and a girl—tragically getting run down by a speedboat one summer afternoon, with a sudden eight-year time jump obscuring just who exactly survived the case of water skiing gone horribly wrong. After the flashforward, young teen Ricky (Jonathan Tierstan) and his mostly silent, cripplingly shy cousin Angela (Felissa Rose) are packed off to sleepaway camp by the woman who raised them both, played by Desiree Gould in the style of Johnny Depp‘s Willy Wonka on unhealthy amounts of cocaine. Almost immediately after the kids arrive, people start dying in horrific ways, all of them coincidentally people who happened to mess with the bashful Angela. These events don’t so much build as they do tumble toward the final 30 seconds, in which it’s revealed that “Angela” is actually Peter, the boy survivor of the opening speedboat accident, standing naked above a newly headless corpse with his male genitalia on display and horrific animal noises coming from his mouth.

To clarify, it’s not the twist itself that’s scary. Despite that one unnecessarily yoked camp counselor’s cry of “My God, she’s a boy“, the sudden revelation that Angela had a penis the whole time isn’t going to cause any nightmares. The terror of Sleepaway Camp‘s ending comes completely down to the image itself, which horrendously combines Lynchian uncanny valley with the human brain’s ability to just know when something isn’t right, even if it can’t specifically put together why. It’s not even clear if the effect is intentional or just a result of 1980’s filmmaking technology. To pull off the twist, Hiltzik superimposed an un-moving image of young Felissa Rose’s face—frozen in a rictus somewhere between horror and joy—on to a decidedly non-childlike muscular male body, resulting in something undeniably not-human looking. It’s like the most fucked up optical illusion you’ll ever see, the visual cues of what makes a person a person all wrong in unexplainable ways, an action figure put together by a child with only a basic understanding of the human anatomy.

Nothing helped to explain my feelings toward Sleepaway Camp‘s ending better than writer Sean T. Collins‘ brilliant visual essay on the “Monumental Horror Image.” You can check out the whole thing here, but this passage, in particular, struck a chord while thinking about Sleepaway Camp‘s final image:

The things you see in images like these aren’t brandishing a chainsaw or baring a mouthful of fangs, but something about them feels completely terrifying anyway. It’s not just scary, it’s wrong, like you’re seeing something that should not be…Why “monumental?” In part, because subjects of these images are horrifying more for what they represent than what they actually do. In most cases, they don’t do anything but stand there.

Because boiled down, what makes the end shot of Sleepaway Camp so jarring is that it’s wrong. That’s not what a human being looks like, male or female. Those noises coming from Angela’s unmoving mouth, those are not human noises. And by ending on such a striking note of indescribable wrongness, Hiltzik paints the comparatively boring ninety minutes that preceded it in a different light. Is there something we missed about Angela this entire time other than the fact she’s a boy? Something that would explain why, without clothes, she’s something closer to a wood-beast from Lovecraft’s worst fever dream than she is to a human being?

Or, equally as likely, it’s a happy accident for all involved with the making of Sleepaway Camp, the fact that their shoddy FX work churned out something that looks like a member of Motley Crue was exposed to gamma radiation, elevating an unearned and frankly problematic twist into something more cerebrally terrifying. It’s hard to tell, just like it’s hard to argue with the fact that Sleepaway Camp‘s ending is still just as hard to look at without fingers in front of your eyes even 35 years later. But like all of horror’s most enduring unexplainable imagery—Pennywise in the sewer drain, Regan floating above the bed, two murdered twins at the end of an Overlook Hotel hallway—Angela unclothed at the end of Sleepaway Camp is equally hard to look away from.