Accepting only the comfort of her headphones, high school student Hikaru Takabe spends her time isolated from her classmates. However, her life takes a new turn when she meets a powerful intergalactic being named Horizon. As a result of the surreal incident leading to their encounter, Horizon must stay within Hikaru’s body, much to her chagrin.
Now armed with Horizon's powers of spatial manipulation, Hikaru is dragged into the mission that brought it to Earth: to destroy Maelstrom, a lifeform whose instinctive purpose is to kill every living being on whatever planet it finds itself on. For Hikaru, this not only involves partaking in an evolutionary game of cat and mouse, but also taking the first steps out of her shell.