The entire movie felt like the prelude to some greater universe . . . the first act of a bigger story. It throws a bunch of puzzle pieces on the table, interlaces them with various pieces from different puzzles, and tells you to figure it out. I wanted to love it, but it's a legitimately bad movie, despite all of its very high quality components I mentioned above.
The director has said that he knows in his mind everything that happened off-screen. Well, that's great for him! But he didn't show us, or give us nearly enough clues to paint a coherent picture. I'm someone who loves slow burns, and movies you have to think a little bit to get, but this movie isn't that. The title alone is the first and most important lie in this marketing campaign, made even worse by the disingenuous trailers I've since looked at.
"It Comes At Night"? Well, no, "it" never does. There is no "it" in the movie. Could the movie have conceivably gone with a more abstract interpretation, like paranoia? Sure. But if you ask 20 people coming out of this movie what "it" is, their answers will be as amorphous as what Bill Clinton thinks "is" is. And that's poor movie making.
Why do they absolutely never go outside at night? That implies more than a natural fear of the dark. It implies something is out there.
What does the dog chase after? What does Travis see when he finds the dog, assuming that wasn't a dream? What kills the dog, as is implied by the audio? How did the dog get back to the house? Who opened the doors? Why was the child asleep in the other room, and how did he get there?
What the fuck were the dreams? Were they "it" (surely they would be "they." lol)? At what point was Travis infected? Were the dreams the result of his incubating infection?
To say nothing of what on earth the infection even is.
As best I can piece it together, and this is being generous:
Travis goes out searching for the dog. He finds the dog dying violently of its last stages of infection. He brings it back to the house, and leaves the doors open. He's contracted the virus from the dog, and transmits it to the child when he wakes him up (presumably the kid was just sleepwalking, though his parents denied any history of it). Rather than absconding in the night when Paul and family are quarantined and stealing supplies, Will ridiculously plans to ambush Paul and co. with this gun he's somehow smuggled in. Yada yada, husband and wife choose to comfort Travis as he dies of infection, succumbing to infection themselves rather than live without their son.
That's probably not, actually, what happens, and we can't reasonably conclude that it is. But it's my best guess with the nonsensical pieces we're given. And it doesn't excuse any of the movie's overt implications that something is out there.