Miu Amano owns a box that her father gave to her five years ago just before he left. Every year on her birthday, he sends her a postcard, but this year is different. This year he sends a present—a key that fits the box's lock perfectly. Inside the box is a small device that transforms into a tiny cat that calls itself "heaven's cat," Francois d'Mielle the third. He tells Miu that her father gave him an assignment—to turn Miu into an angel.