Mutsuki Nenga serves in the Galactic Patrol, hailing from an alien species with nitrogen-based biology; exposure to water proves lethal, and stress triggers mushrooms to sprout from her scalp. She operates alongside her younger brother Masatsuki, both driven to restore their family's honor after their father, also a Galactic Patrol officer, faced disgrace. Tasked with capturing the space pirate Mito, she enrolls undercover at Aoi Mitsukuni's school as a transfer student to gather intelligence. During this mission, she develops romantic feelings for Aoi, creating internal conflict between her duty and personal emotions, particularly as she investigates his connection to Mito.

Her relationship with Aoi evolves significantly after he undergoes an involuntary gender transition due to biological traits inherited from his mother's species, stabilizing as female by the series' conclusion. Despite this change, her affection persists, navigating complexities arising from Aoi's new identity and role as galactic queen. Later, she becomes possessed by the former Queen Hikari, who exploits her body as a vessel to consolidate power, though she eventually regains autonomy. Following this possession and Hikari's defeat, she remains aligned with Aoi's faction, supporting the transition from monarchy to democracy. Her narrative involves reconciling familial loyalty with evolving perceptions of justice, especially as her brother experiences psychological turmoil over ideological conflicts.