When Mimii was young, she was friends with Hoshiko. Hoshiko had to go to the United States because she was studying abroad. Mimii makes Hoshiko promise that they will always be friends. And this is where the story starts.
Meca shows up in Mimii's life 7 years later to become her friend, and to help her. However, Meca isn't exactly all that Mimii thought she would be. Meca keeps getting into trouble, saving people's lives, and summoning various people/robots/objects with her cellphone.