A.R.M.A—Autonomous Robotic Military Arsenal—was created to serve as war machines without human involvement. However, nations used them to invade and conquer, igniting the Third World War, known as the A.R.M.A Conflict. After five years, the most powerful A.R.M.A, capable of hacking others, turned into the self-aware entity "Armagedoom," which seized control over all A.R.M.A units and turned them against humanity.

Six years after the A.R.M.A Conflict, a mysterious young girl named "Smile" emerges from the wreckage of an abandoned laboratory. A fusion of human and A.R.M.A technology, she possesses little knowledge of her origin but knows her mission: to destroy Armageddon and save an an unknown friend.