Himeno's chest will hurt if any boy touches her because she has the diamond of the heart. However, when she was young, she befriended two boys -- the gentle Mamoru and the blunt Keigo -- who are able to touch her without harming her. Without knowing about the diamond of the heart, both Mamoru and Keigo promised to protect Himeno. When some people threaten to take Himeno's heart, who else will protect her but her knights? (Source: MU)