When a Category 5 tornado ravaged Mercy, Oklahoma, no family in the small town lost more than the McClouds. Their home and farm were instantly demolished, and orphaned siblings Darlene, Jane, and Cora made media headlines. This relentless national attention and the tornado's aftermath caused great tension with their brother, Tucker, who soon abandoned his sisters and disappeared. On the three-year anniversary of the tornado, a cosmetics factory outside of Mercy is bombed, and the lab animals trapped within are released. Tucker reappears, injured from the blast, and seeks the help of nine-year-old Cora. Caught up in the thrall of her charismatic brother, whom she has desperately missed, Cora agrees to accompany Tucker on a cross-country mission to make war on human civilization. Cora becomes her brother's unwitting accomplice, taking on a new identity while engaging in acts of escalating violence. Darlene works with Mercy police to find her siblings, leading to an unexpected showdown at a zoo in Southern California.