In the near-future, the world's energy problems have been solved thanks to a scientific power known as the Manifestation Engine which provides energy from the sky. On the island of Izu Oshima, Akane Isshiki is living a simple life with her sister, Momo, and her grandfather and creator of the engine, Kenjirou. As Akane eagerly awaits the arrival of her classmate, Aoi Futaba, who is travelling to the nearby Blue Island for treatment, she and Momo hear an explosion coming from their home and discover that Kenjirou has somehow switched bodies with an inanimate ferret doll as the result of an experiment. Undeterred by his new form, Kenjirou gives Akane his latest invention, a strange key, in order for her to fight against a strange alien mech, known as an Alone, that is attacking Blue Island, where the Manifestation Engine is stored. As Aoi's transport is hit and crash lands on a ledge, Akane and Kenjirou rush to their location, but Akane is soon struck by her fear of heights that she gained during a traumatic incident. When Aoi starts to fall towards the sea, Akane overcomes her fear and dives after her which activates her key, giving her a special suit known as the Vivid System which stops Akane and Aoi from falling. Kenjirou explains that this system is for Akane and Aoi to fight against the Alone.