“When that rain of stars falls, a miracle will surely come to you.” On the day the meteors rained down, his mother vanished. Years later, ordinary high school boy Hoshizora Amamiya is suddenly visited by a mysterious beauty, Shiho, who appears together with a falling star. She abruptly declares that she will protect him from a looming alien threat, and before he knows it, the two of them are living under the same roof.