Four waves of increasingly deadly alien attacks have left most of Earth decimated. Cassie is on the run, desperately trying to save her younger brother. – IMDB
The movie opens with a teenager (Chloë Grace Moretz) exiting the woods and approaching a gas station. She is carrying an AR-15, so this is either a post-apocalyptic society or the Deep South. She starts looking around for supplies and stumbles upon a guy who is wounded. He’s asking for help, but our presumptive heroine seems pretty nervous. He’s reaching for something, is it a gun? Too late, she shot him. Turns out it was a crucifix.
This movie is called The Fifth Wave, so we clearly need a recap of the first four (since we don’t know them). Aliens show up unannounced and hover silently over major cities, just like in other movies you’ve seen (Independence Day, District 9). Ten days later a massive EMP hits the whole Earth (presumably) and knocks out all the power. Planes fall from the sky (a la Revolution). This is the First Wave.