The opening scene starts with Cassie, armed with a M16 , in the woods. She emerges and sees a mini store. Upon entering, she hears a male voice calling for help. She follows the voice and enters a room, to find a man pointing a gun at her. She tells him to put his weapon down, and he does so. After this, he in return tells her to drop her weapon. She is wary, because she sees his right hand in his jacket. She demands his right hand to be exposed. As he takes out his hand, Cassie sees a glint of metal, and shoots him to death, in fear that it is a gun. A crucifix on a chain turns out to be what he was holding. Cassie gasps in shock of what she had done before the screen cuts to black.
Cassie gets separated from her five year old brother, Sam. Her brother is taken by the Army for care and protection. Her father and other survivors in Ohio are killed in a massacre by the army. Cassie scavenges her way toward the base to find her brother. She is saved from an enemy sniper by Evan Walker. Cassie falls in love with him but later learns he is an Other in humanoid form. She also learns that Colonel Vosch and the Army are actually the Others and the fifth wave they have planned is to use children to eliminate the rest of the earth’s survivors by misleading them.
She learns the hard way to trust no one and leaves him. The only thing that keeps her going is the hope that she can save her brother from the “training” camp established by the Army (Others). Meanwhile Ben Parish, Cassie’s crush before the wave and her brother are in the same squad trained under the Others. Ben also comes to learn the Others plans. Evan who follows Cassie helps her and Ben to save Sam and demolish the training facility. The Others evacuate from the facility with the intent to deploy the children as soon as possible to other cities. Ben and his squad is seen enjoying their meal with Cassie and ponders hope as humanity’s driving force for survival.