Horror movies are pretty much synonymous with monsters. The definition of "monster" is pretty broad, but there's a Big Bad Something, be it animal, vegetable, or mineral, and it means people ill, and people object to that with varying degrees of success. In pretty much every instance, the monster is at the center of the movie - the movie begins just before we meet the monster, the monster is fought and defeated (or is it?), and it ends with some kind of resolution, either a happy ending or not. There's a plucky band of protagonists, and it's up to them to stop the monster. People are threatened, and then saved. There's a threat, then there isn't, unless it comes back.
At least, that's the convention. As the convention gets reconsidered, we get movies where the monster gets moved to the periphery to different degrees - The Host is nominally about a giant mutant tadpole looking thing, but it's mostly a family drama, in which the defining tragedy is the kidnapping of the youngest daughter by said giant mutant tadpole. Cloverfield is a verité riff on the giant-monster movie in which we don't follow the plucky band of protagonists - instead, we follow one of the groups of people who run screaming from the monster. The Mist is as much about the horrible things people will do out of fear as it is about the gargantuan monstrosities lurking outside. Monsters move from being the driving threat to being the catalyst for other events.
Monsters takes this one step further, by making monsters a preexisting condition, a fact of the world.
We learn from a title card that six years ago, a deep-space probe tasked with searching for extraterrestrial life blew up on reentry, scattering alien life all over a large swath of Mexico, This life has flourished at the expense of the native flora and fauna, and Mexico is cut in two by a heavily-patrolled "Infected Zone." This is just a fact of the world now. Children incorporate the aliens into their murals, immigration is locked down even more tightly than it was before, a mammoth concrete wall (visible from space) runs along the U.S./Mexico border. The events of the monster movie have happened, and the alien invasion was not averted. They are here.
Against this backdrop we have the story of Andrew and Samantha. Andrew is a freelance photographer, trying to get into the Infected Zone to get photographs of the aliens at close range - highly prized, these would be a big payday for him. The closest he's been able to get has been pictures of the monsters' wreckage - destroyed vehicles and buildings, cars and airplanes flung up into trees, the carcasses of dead aliens. His window of opportunity is closing because the U.S. is extending a program of carpet-bombing further out from the Infected Zone proper in an attempt to control the alien population. It's also creating tens of thousands of refugees as Mexicans are driven from their homes. He's going to have to get out of this part of Mexico soon, so he wants to get into the Infected Zone before he has to go back home. His one shot is interrupted by a call for him from the publisher of the magazine for whom he is shooting. The publisher's daughter Samantha is lost somewhere further south - injured and staying in a Mexican hospital. Andrew is tasked with bringing her back to the U.S., his big chance be damned.
Naturally, there's some resentment from both ends. Andrew is mad that he's lost his one shot at the big time to play babysitter, and Samantha has to put up with a bit of a boor in Andrew. She speaks the language, he doesn't. She's concerned about the people in the villages, he's looking for the perfect shot. She wants to help, he wants to party and be done with his errand as soon as possible. This could tip very easily into romantic comedy cliché, but the actors play it just low-key enough to keep it on a human scale.
Getting out of Mexico ends up being more difficult than either of them thought - lines of transportation are limited, and the people controlling them are making a killing off the people who need to get out. It's the last day they can run the ferry before the military moves in, so the tickets going for thousands of dollars. Andrew and Samantha give everything they have to someone who is one step above a smuggler, and there's no guarantee that he isn't just going to leave them for dead, but they have no choice. The Infected Zone is the dangerous unknown lurking miles away, but Mexico itself feels just as predatory. There's a sense that these two people are innocents abroad, and the journey is going to be just as scary (if not scarier) than the monsters.
One thing leads to another - many shots of tequila and regrettable decisions during their last night in Mexico, and they wake up in the morning without their ferry tickets or much of anything else. The ferries are through running, the military is coming in. They have to leave now, and there's only one way left to go: Through the Infected Zone.
Of course, gringos horrified at something in the Third World is not a new thing, and the parallels with issues like immigration, foreign policy, and colonialism drift into the ham-handed on a couple of occasions, but like Andrew & Samantha's changing relationship, generally stay on the right side of believable. They're brought together by ordeal and fear, and the need to cling to something familiar. They meet halfway through difficult circumstances, and this is mostly the focus of the movie, rather than the monsters themselves. This becomes less true toward the last act, where danger is equally intermingled with beauty, relief with despair, closure with uncertainty, and a scene at the film's open is revisited from a different perspective, neatly bringing the whole thing to a close. I'd hesitate to call this a horror movie - more of a "quietly upset" movie - but it's an interesting addition to the increasing vocabulary of the monster movie, all of its typical bombast and shouting brought to the level of a conversation between people.
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