Kurenai Mishina transforms into the superhero Otomet Amarilla, linked to the red shooting star. Following the deaths of her parents, Dr. Mishina and Dr. Kishida, she and her sisters Aoi (blue) and Midori (green) inherit three of the seven fallen stars and the specialized Musumet helmets designed to study and control them. These helmets amplify the sisters' inherent physical and mental abilities to superhuman levels. Kurenai and her siblings enlist in MET, the secret organization founded by their parents, to defend Earth against threats from the remaining four stars and safeguard global peace.

Operating as the Otomet Amarilla, Kurenai fights alongside her sisters. Their Musumet transformation equips them only with helmets, gloves, and boots over their existing civilian clothing—whether school uniforms, gym suits, swimwear, or, depending on the suddenness of their deployment, even while nude. They undertake heroic missions in these everyday states while simultaneously navigating typical adolescent lives filled with school, social interactions, and romantic interests. Their activities frequently bring them into conflict with the wealthy, arrogant Saotome siblings: Marcia, Kou, and Shion.