In pursuit of an alien fugitive named Geega, Birdy Cephon Altera, a spirited officer of the Space Federation, finds herself on Earth. Geega has assumed human form and slipped into the fashion industry, prompting Birdy to adopt a new identity as "Shion Arita" and join a modeling agency. During the day, she works as a promising model, while at night, she continues her mission to capture intergalactic criminals.
Concurrently, Tsutomu Senkawa, an average high school student, and a friend explore an abandoned building, unknowingly converging on the same site where Birdy is tracking Geega. Tsutomu briefly sees them fighting until Geega takes him hostage. In her assault on Geega, Birdy inadvertently kills Tsutomu but, overwhelmed with grief, merges his consciousness into her own body to save him.
As a result, Birdy and Tsutomu must coexist within one body, managing Tsutomu’s high school responsibilities, Shion Arita's burgeoning modeling career, and Birdy's escalating tasks as a Federation officer.