While walking back from school to your home with your best friend Ishirada Riko, you both see something weird in the sky.

"W-what is that...? It looks like... a UFO?!"

Suddenly, the object flies closer to both of you, and before you can react, a weird green light emanates from the UFO, causing Ishirada to disappear in front of you. The UFO then vanishes, and two days later, Ishirada becomes one of the disappeared people. While you are alone at your house, the doorbell rings. When you open the door, you are surprised to see an unknown girl.

"H-hey! I am Ishirada Riko... your friend... the alien turned me into a girl!"