This overpoweringly tense and deeply mad thriller from Colombian film-maker Alejandro Landes is the best thing I have seen at Berlin this year: something between Apocalypse Now, Lord of the Flies and Embrace of the Serpent. It depicts a dysfunctional society and guilt-ridden family in miniature, and demonstrates the shifting power dynamics of a cult, the craziness embedded in the minds of child soldiers, the resentments nursed in a military unit without a supervising commanding officer, and the very real danger of eating shrooms grown in cowshit.

The Monos are a unit of teenage guerrilla bandits, operating in a country very similar to Colombia or Bolivia — and apparently named after the Mono Grande, the mythical giant monkey for centuries rumoured to exist somewhere in South America. They are initially shown in their up-country mountainous retreat where they have little to do but hang out, await orders on the radio, look after a cow that has been given to them for milk (good for their young bones and teeth) and see to their American hostage, an engineer who is being held captive separately from her small child. They are permitted and even encouraged to develop sexual relationships among themselves, and have developed weird rituals and traditions. When one of them has a birthday, there is a version of the bumps, except it’s sexualised thrashing with a belt. Everyone joins in and their hostage is also sportingly allowed a go.

But what they most like doing is firing their semi-automatics, an excitement simply impossible to resist, and this, inevitably, leads to disaster. (I was reminded of something PJ O’Rourke wrote about hearing semi-automatic fire for the first time at close quarters: the “chemical craziness” it triggers in the pit of your stomach – something that is horrible for most, but for some could be addictive.) The Monos are required to move from the eerie aloneness of the mountain down into the jungle, and when the hostage shows signs of wanting to escape, the leader has a spasm of rage, trashes the radio that theoretically connects them to a larger command structure and the entire group effectively goes rogue – even more rogue than it was in the first place.

It is not clear if they are now just breaking the rules in pursuit of some definite objective, or submitting to some collective orgiastic breakdown. There is no Kurtz, no progressive loosening of the unstable bonds of civilised restraints; it’s just an intensification of the chaos which was there all along.

Monos is not concerned just to show a static situation deteriorating. There is a driving narrative impulse: tense things happen. And they are made even tenser by the vivid, disturbing musical score from Mica Levi, which periodically rolls in timpani like exclamations of thunder.

Jasper Wolf’s cinematography captures the transcendent beauty of the landscape, the density and hostile lushness of the undergrowth and the banks of cloud with us floating overhead, as if monitoring the characters’ habitat in some dreamily removed state.

Facebook Twitter Pinterest Transcendent beauty … Monos. Photograph: Lifestyle pictures/Alamy Stock Photo

What is so intriguing about Monos is that it is simply about the group, collectively, with no member more important than any other. Individuals do emerge, yet it is never clear who is evolving as the most significant. Landes’s film refuses the final girl trope or indeed the final boy trope. And of course it is not simply about child soldiers, but rather soldiers generally, although it is conceivable that their parodic littleness offers a faint pun on Conrad’s Mr Kurtz: sounds like kurz, or short.

In some ways, Monos is a ritual heading to disaster, like a Jim Jones ceremonial in the jungle clearing, or alternatively a dream, a hallucination, one that we are having about them, or they are having about themselves: a delirium brought on by altitude sickness or hunger or post-traumatic stress disorder. And Landes withholds from us any sense of when precisely this is supposed to be happening: now, or in some postapocalyptic future or in the past, as if his children are a version of the mythic Japanese soldier in the jungle unaware the war is over.

Perhaps it is an allegory about his homeland, Colombia, a country awakening as if from an endless dream of violence but with an uneasy suspicion that the seedlings of new violence are always there. An unforgettable immersion in terror.

•Monos is released in the UK on 25 October