Ippan Saibou envies the acclaim given to immune cells such as white blood cells for defending the body against lethal pathogens. Rather than staying safely at home, he longs to contribute and earn acknowledgment for his deeds. He soon stumbles upon a cluster of juvenile‑appearing bacteria being swept away by a river flow. Though aware of the risks they pose, he saves them and shelters them safely. Yet Hakkekkyuu U‑1146 detects their presence and begins a pursuit.
Elsewhere inside the body, NK Saibou is searching for someone. According to her information, he is an antigen the immune system has encountered before and may have returned for vengeance.
