A bunch of mysterious stuff happens. The big reveal at the end, of why the mysterious stuff was happening, is interesting, frightening, and almost plausible. So if you are able and willing to disregard the movie's many flaws up until that point, you will enjoy it.
But the flaws were numerous.
There were far too many weird things going on and not enough explanations. And I was thinking that at the beginning, not the end. I was thinking that, even if they come up with a perfect explanation for all this stuff, there's still too much stuff going on. When there was just 15 minutes left in the film I knew we were screwed, because it would take at least 30 minutes to explain all the weird stuff that went on. The movie couldn't decide if it was trying to be horror, thriller, or whatever that magical mystery genre is that includes the TV show Lost. This film reminded me of how I felt when I watched a single episode of that TV show. I never watched another because I could tell that with that level of weirdness, the only possible explanation would have to be so ridiculous that it would be completely unsatisfying. "It was all a dream," or something like that. As I mentioned, this film actually had a good wrap-up-premise. But the way it wrapped things up was just by never attempting to explain all the weird stuff that they put in there to try to make the movie weird and mysterious. It just left a bunch of weird stuff with absent or unsatisfying explanations.
I'm just going to call it a thriller, because that's what it's best at. It should have given up on the horror and halfhearted surrealism.
Now for some examples of the excess unexplained weirdness.
1. Aside from waging a large-scale conventional or nuclear war, humans would be hard put to do anything that could disrupt the lives of so many species of animals as dramatically as they did in this film. In this film, flamingos manage to fly from Florida to Long Island in 48 hours. Deer keep gathering and surrounding humans for no reason. Deer are not particularly threatening, especially when you're a hungry human in the apocalypse. They were supposed to be scary but they just looked delicious. What were the deer even doing? What did they want? Hitchcock managed to make birds feel threatening but this director is no Hitchcock.
2. In this film, airplanes and boats randomly crash when someone hacks into their computer systems, but these types of crafts have backup systems in place so that when electronics fail they don't crash.
3. A dude's teeth fall out randomly, or strictly speaking after a tick bite, but still, that's pretty random. No disease makes your teeth fall out in just 48 hours, and if you got a disease that mysterious, no doctor would have any idea how to cure you, let alone the survivalist who they go to visit to demand medical assistance from. And there's no way that the cure for a disease that mysterious would just be a pill that humanity had already invented. But the survivalist sold them some pills and I guess that was it, the kid was now cured.
4. Random screechy noises came from the sky. It was suggested that they might be noise weapons. Noise weapons exist but no weapon that loud could be used from a great distance due to the inverse square law. You'd have to blanket the country with these things. I'm pretty sure these weapons are moved around on trucks. So the noise weapon attack was implausible.
5. Long Island was weirdly deserted but that was never explained. It was just a shoehorned-in thing to support the movie's title, the idea that they were far away from humanity. And being far from humanity actually ended up benefitting the protagonists, rather than being a way to up the stakes. If the film is going to end tragically, or semi-tragically, as I think it did, its title should reflect one of the threats they faced, rather than one of the refuges that helped ameliorate those threats. In fact, hordes of terrified people could have served to make the movie more ominous.
6. In one scene, paper flyers are dropped over a deserted highway in a propaganda campaign. That's not the best use of your limited flyer-dropping resources. That much paper would have weighed several tons. You'd think they'd drop that many flyers over a city.
A simpler plot would be better. Simply not having electricity across an entire country for a long enough time would cause just as much drama. They just added the other stuff in because they were trying to make a longer and more mysterious movie. And yet it took forever to raise the stakes. Halfway through the film they still had power in the house.
People tend to work together in crises, at least up until they start to starve. But this film tries really hard to focus on, and caricature, the tensions between the characters. The point it was trying to make is a good one, though. Those tensions that do exist, would not serve us well during a crisis, if we were to let them affect the rational decision-making process. But the movie tensions themselves felt forced. For example, the white characters have doubts about the black characters' motives. They suspect them of trying to run some scam. But the black characters showed up in a Bentley, after a night at the orchestra, in black-tie attire. I've known a lot of white people, but I've never known any that were so racist they'd even suspect these clearly wealthy black characters of running some kind of scam. The movie pointlessly tries to draw out this ambiguity as long as it can when the black character is unable to prove his identity by presenting id, due to having lost his wallet. The tension went nowhere because it ultimately turned out that yes, these characters were exactly who they said they were. In a whodunnit it might be fine to have some red herrings like that, but in a thriller you expect a slow reveal of secrets, without nothingburgers that never pay off.
SPOILERS FOLLOW
The 13-year-old girl character randomly leaves her family. Why? Even 13 year olds have semi-logical thought processes. The girl mentioned a story she'd heard, about God giving humans hints, but humans foolishly choosing to ignore them. But that story simply didn't apply to the events in this film. The girl complained that no one listened to her, but she didn't saying anything that was of any use to anyone. None of the characters had any idea what was going on, including her. She was acting as though there were some obvious solution that everyone was ignoring, but at the end when she found a solution, she merely stumbled upon it. I think the other characters eventually converged on the same solution through more rational means, but the movie ended without making that clear.
The 13-year-old girl had a desire to watch the last episode of the TV show Friends. She'd been watching it when the internet cut out, at the beginning of the film. So when she found an emergency bunker with a DVD collection including all of the episodes of Friends, it was supposed to be some sort of a victory for her, and her strategy, I guess. But as I said, her strategy was to offer nothing, complain that no one listened to her, wander off alone without notifying anyone, and then merely stumble upon success.
The film ends without even making it clear who survives, and how. After all the drama and buildup, it could have at least tied up that question more cleanly. The annoying chapter headings that appear throughout the movie called the last chapter "The Last One," which implies that the character the camera settles on at the end, the 13-year-old alone in a bunker, might be the only one who survives. But if that's supposed to be the case, this is a very dark film. But if it's a dark film, it should have leaned more in to that. It could have been darker. We could have witnessed how the characters' foolishness led to their deaths. And it would have been darkly funny if the girl discovered the DVD collection, but she had no idea what a DVD was or how to play it. It would have been like that Twilight Zone episode where the guy who only wants to spend his life reading alone, thinks an eternity stuck in a library is paradise, until he discovers that his glasses are broken.
So I do not recommend Leave the World Behind. What you should watch instead:
Don’t Look Up (2021) or Magnolia (1999). Don’t Look Up is a dramamtic comedy, but it does a better job of highlighting how peoples’ differences could stand between them and success. Magnolia, like this film, features strange mysteries, and a masterful buildup of tension between several characters. Yet it is far more profound and emotional.