Miu's father left her five years ago with a box. The box was locked, and Miu's father took the key with him. on her birthday, he sent it back, and inside the box was a mahou shoujo transformation device! Now it's Miu's job to transform into Shoujo Tenshi Milcute and save the world!
EDIT: The author put this series on hold and later cancelled it because she wanted to start Modotte! Mamotte Lollipop, and later Navi Luna.