Years ago, a village boy named Duanmu Yeyu was rescued by the Hell Hunters, the elite protectors of Ember City, and he has wanted to join their ranks ever since. His trip to the city to chase that goal pulls him into trouble he never saw coming. The villain Lucifer has sent MIKO-Taiyi, a bioengineered agent, to slip into Ember City and steal secret data, but Taiyi is badly wounded in a clash with the Hell Hunters and collapses. Yeyu finds the unconscious operative and, with no idea who Taiyi is, brings Taiyi to the Hell Hunters' base for care, carrying off a vital mission device along the way. As Yeyu starts his trial to join the Hell Hunters, Taiyi must quietly get close to him and recover the device without being caught by the city's forces. Meanwhile, Lucifer sends tens of thousands of minions to hunt for the missing agent, and one kind gesture grows into a conflict that puts all of Ember City at risk. The 15 episode ONA mixes action and science fiction.