In an apocalyptic world where a god-beast named Colossus wages war on the human race, a group of teenagers named the Fireflies are tasked with fighting Colossus before he's given the chance to eliminate mankind for good. The Fireflies fight inside mechanical robots termed mechs. They're offered as sacrifices by the surviving adults. Their deaths aren't important; what's important is that they rid the world of Colossus. The teenagers are ordered by an abusive colonel named Beckford, who will never be satisfied until Colossus is killed. Signal is a rebellious Firefly. Although she has gone up against Colossus seven times, each time she has neglected to kill him, resulting in Beckford burning a cigarette on her arm. As Colossus approaches this eighth time, Beckford commands Signal to get into position, but this time, Signal has other plans. Using peppermint spray and music, she lifts herself out of her mech and pacifies the creature. They walk away from the shores into the mountains along with the other Fireflies, abandoning the adults of the colonies to fend for themselves in the violent, apocalyptic world.