One day, the Earth, the home of mankind, was attacked violently by aliens. Modern facilities and advanced technology suffered a devastating blow, and human civilization seemed to have returned to the Stone Age. Casey (Chloe Grace Moretz) and her brother Sam (Zackary Arthur) lost their mother in the disaster. The two siblings did not care about their grief and followed their father Oliver (Ron Livingston) to the refugee camp for refuge. Colonel Voss (Lev Schreiber) led the army to the refugee camp. They took the children away and claimed that they would come back to pick up the adults left behind. Casey and Sam reluctantly left their father. Sam's beloved teddy bear fell in the refugee camp. Casey returned to find it, but she didn't know that the aliens had the ability to live in humans, which caused unrest in the refugee camp. In the chaos, Casey and Sam were separated. In order to find her brother, Casey resolutely embarked on a dangerous journey.