Yui is a young schoolgirl with a brother complex. One day she witnesses Serina Shidou kill someone in the park. It turns out he had turned into a demon!

Serina now insists that Yui become a magical girl. If she refuses, or if she won't fight demons Serina will simply murder her.

So Yui becomes Magical Girl Miracle Yuiito and fights demons while Serina's heart softens toward her new friend and humans in general. But how will they fight back when the enemy knows who they are?