Wyatt is convinced everyone around him is turning into monsters, because of mysterious phone calls he receives in the night. Calls that tell him he's going to have to fight back, that he needs to prepare, needs to go hide somewhere in the wilderness.
Instead, he runs into his old friend Christian, and stays at his apartment in New York City while he tries to decide what to do. Christian has gone through some changes himself, trying to remake himself to be more assertive and successful at work and in his personal life. Which means that after a while, Wyatt starts to wonder about Christian, while also wondering about whether he's nuts or not. I spent the entire movie unsure. Heck, I'm still not entirely certain.
It's mostly about these two guys and their attempts to deal with their issues. Wyatt broke things off with his fiance recently, because she cheated on him, or he thought she was one of the monsters. But there's a brief exchange with a psychiatrist where the doctor asks if Wyatt's ever had a relationship where the other person didn't let him down. So Wyatt perhaps doesn't deal with change well, people behaving outside the boundaries he sets, and his brain.
Christian's girlfriend broke up with him, and so now he works out, and reads online articles about how to be more confident, or how to ask out your boss (which just seems like a tremendously bad idea). He tries to make himself into something he thinks will be more successful, because the person he is isn't enough.
The parts that resonated with me were them trying to reconnect, do the same things they did when they were younger. Each of them is different from how the other remembers, and each is trying to figure out how to be the good friend right now. Encourage the other to go out, or stay in and goof around? Keep your own troubles to yourself? Also the way Christian is altering his normal patterns to accommodate Wyatt's presence. He's like this new object in a box stuffed full, and everything else is getting jostled around to make room. And the fact that, when Christian is at work, Wyatt is slightly at loose ends. There's only so much time he can, or wants to, spend contemplating or preparing for the upcoming conflict, but he's on his friend's turf. What to do with himself?
The film has this signal that Wyatt's sensing the presence of the monsters, where all other sounds drop away, and there's the persistent buzzing of flies. It's a clever approach, if they want there to be doubt about Wyatt, his fixating on what is typically an innocuous sound is a way to do it. There are flies around all the time, which could play into the notion the monsters are increasing in number, taking over more people. Once he becomes fixated on them, he's more aware of the noise, which feeds into the conviction.
Or there really are that many more monsters. In which case, he and Christian are going to need a lot more acid and power tools.