Ohio high-schooler Cassie Sullivan, armed with an M4 carbine, emerges from the woods to raid an abandoned gas station. Upon entering, she hears a male voice calling for help. She finds a wounded man, who points a gun at her, but they ask each other to put their respective weapons down. His other hand is under his jacket—and as he takes it out, she sees a glint of metal and kills him, thinking he was holding a gun, but he was in fact holding a Christian cross; the screen cuts to black to show her backstory.
A colossal alien spaceship is circling Earth, guided by extraterrestrials referred to as "The Others". Ten days later, The Others unleash their 1st Wave, an electromagnetic pulse that permanently disables all electrical power grids worldwide. The 2nd Wave has The Others manipulate the planet's geology and fault lines, causing earthquakes and megatsunamis that destroy coastal cities and islands, including Hallandale Beach, London, Bangkok and New York. For the 3rd Wave, The Others modify a strain of avian flu virus and spread it across the planet with birds. The population is decimated, with Cassie's mother being one of the casualties.
Cassie, her father and her younger brother Sam find a summer camp in use as a refuge in the woods with roughly 300 survivors. A few days later, an Army unit with working vehicles rolls into the camp. The unit's commander, Colonel Vosch, claims there is an imminent threat of a 4th Wave and they will take the children to safety at Wright-Patterson Air Force Base, intending to bring the buses back to retrieve the adults. Cassie is separated from Sam, and witnesses the Army massacring all the adults, her father included. Cassie scavenges her way toward the base, but is shot in the leg by a sniper and passes out. About a week later, she wakes up in the farmhouse of a young man named Evan Walker, who saved her. Cassie leaves towards the base with Evan, but learns that he is an Other, sent years ago as a sleeper agent and merged his consciousness into a human host. The sleepers roam their designated zones, killing human survivors. Evan admits his humanity was reactivated when he saw her, disagrees with the invasion and lets her leave. He warns that Colonel Vosch and the military are possessed by the consciousness of individual Others.
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