Japan is decaying. Recession, a rapidly aging population, endemic corruption, and secret slush funds have hollowed out the nation from within. In this atmosphere of decline, secret Earth Defense Force operative Yamato Koichi dedicates himself to capturing extraterrestrial lifeforms, believing that eliminating these alien threats is the key to restoring national peace. His conviction is shattered when he meets a mysterious woman who belongs to an Extraterrestrial Life Force, an alien group whose home world was destroyed. She poses a devastating question: what is it that truly needs to be driven out of Japan? Her words force Koichi to confront a different and far more insidious enemy — the entrenched, unaccountable corruption and the powerful, parasitic old guard that the law cannot touch.

Koichi forms an alliance with this alien woman, known as Piroko, and together they embark on a violent crusade to purge Japan of its "老害" (rougai) — the entrenched, harmful elderly elite. Their first major target is Prime Minister Hayashimichi Akira, whose assassination they orchestrate. As the pair escalate from targeted killings to outright political insurrection, they are opposed by a special countermeasures unit led by Shinano, while a parallel plot unfolds involving an innocent middle school student named Nagito, whose sister's body was stolen by a Transun alien. Nagito's desperate, personal quest to reclaim his sister clashes with Koichi and Piroko's grand, ideological purge.

The story's central conflict comes to a head on the floor of the National Diet building. Three irreconcilable forces collide: Koichi and Piroko as the "night watch" executing the old guard; the Earth Defense Force trying to stop the mass slaughter; and Nagito, who only wants to free his sister. Their confrontation triggers a dramatic change in leadership, as a new national leader rises with promises of a brighter future — words that ring hollow to those who have already witnessed the price paid for this new order. The series explores the slippery slope between patriotism and murder, asking whether a society can be saved by sacrificing the very rule of law that defines it.