In 2119, the spaceship *Green Sleeves* discovers that three planets have been attacked by the roaming planet X‑23, which is moving toward Earth. The Earth government orders the *Green Sleeves* to strike down X‑23, but its captain, Dr. Sugita, refuses, believing that coexistence is possible. After his refusal, the ship’s communications are abruptly cut off. Two years later, in 2121, Susumu, Dr. Sugita’s son, is visited by a group of strangers who escort him to Dr. Miya, one of the few who still supports Sugita’s decision. The majority of the populace, however, labels Sugita a traitor. Dr. Miya recommends that Susumu join White Pegasus, a team of specialized medical rescuers. Their Micronizer System can shrink humans so they can perform treatments from inside the body. Shortly after Susumu’s induction, X‑23 comes within visual range.