I was taken with the first half of When Evil Lurks, a truly unrelenting Argentine film that has been making the hype rounds in the horror community as of late; it even played at Indianapolis’s Heartland Film Festival as part of the horror block earlier this month). It’s a nihilistic and nasty thriller that manipulates known tropes of the possession drama to tell a post-pandemic story with an unbridled willingness to go there with its depictions of violence, particularly against children. You won’t find a more engrossing and unpleasant shocker this Halloween season.

Pedro (Ezequiel Rodriguez) and his brother, Jimmy (Demián Salomón), are poor nobodies living in rural Argentina leading lives of essentially no real value. One night, they hear gunshots in the woods. The next morning, they discover the bottom half of a violently dismembered corpse. Could it have been a mountain lion? No. The cut is too clean, and the gunshots they heard echoed too long the night before. Someone murdered a man and whoever it was is still lurking out in the woods.

The two visit a shack nearby to warn their neighbors, an elderly woman and her two sons. They discover the eldest son, Uriel, is “rotten,” a term used for someone possessed by demons. In this world, blighted by God, such an event is commonplace. Uriel’s family has done everything it can to avoid allowing Uriel’s possessor to jump to another host, and the result is their beloved son’s body becoming a bloated and bloody mass of conscious pus.

Writer-director Demián Rugna allows his cosmology to unfold gradually throughout the narrative, but the gist is this: Demons exist. They possess the minds of humans and animals alike, filling their souls with dark thoughts and tempting them with even darker deeds. There are rules to be followed when encountering a “rotten” person or animal. Seven rules, in fact. By happenstance, the number of God.

These include avoiding electric lights, which create false shadows in which evil hides; avoiding firearms, which only allow them to spread faster; and never speaking outright the name of evil. The particular nature of the demons’ impact on human minds mean that autistic or neurodivergent people are affected differently, too. Perhaps the most important is to avoid animals. Demons possess them with ease and can jump from them to a human mind without difficulty. It’s a well-considered bit of world-building that gives When Evil Lurks unique narrative opportunities and which Rugna exploits with precision throughout the film’s first act.

Unfortunately, Pedro and Jimmy are like most people in their version of reality: They just don’t really see the “rotten” as their problem and work as quickly as they can along with their landowner, Ruiz (Luis Ziembrowski) to dispose of Uriel as quickly as possible. That, of course, leads to an epidemic of possession that destroys their world and all that they love.

There are two really successful aspects of Rugna’s screenplay. The first is that he uses possession as an effective allegory for recent responses to the pandemic, including the presence of rules (which our main characters flagrantly ignore to their detriment) and the sense anyone they interact with could be infected at any time. There’s a moment early on where the brothers make their way into the suburbs to retrieve Pedro’s estranged children that leads to a deliciously bloody “dominoes falling” moments that features one of my personal nightmares as a parent. That’s the other part of When Evil Lurks that works: This film is adamant that children are a vehicle for true terror, whether it be watching them die horribly or watching them kill horribly. When it works, it really works.

It seems pointless to nitpick the back half of the film when the beginning is so gleefully successful at creating a world where anything can happen, and often does, within the central logic of its narrative. As far as it goes, this is probably the best wide-release horror picture of the year. However, I did feel the story start to flag as the back half becomes a slow, dread-filled shamble to an inevitably tragic conclusion. The final setpieces are filled with redundant exposition and payoffs that feel less playful than the earlier part of the movie. There’s an emotional story in the middle act that ends abruptly and deflates the final act, too, and the twist ending is frustratingly understated despite being obvious. It’s the sort of movie that ends five minutes after the ideal closing shot.

Look: As a fan of horror films, it’s hard to say “Listen to me: This one ends up a disappointment” when almost everything building up to the underwhelming finale is such a banger. The poster features a woman about to hit her own face with an axe, and that happens almost first thing. When Evil Lurks premieres on the streaming service Shudder on Friday, just in time for a last-minute Halloween movie marathon, and there’s really not a better 2023 horror release to watch this season.