Working as a nurse, Yuki Mizuho lived a peaceful and quiet life every day. One morning, on her way to work, she encountered a man covered in blood collapsing before her. Despite her shock, Mizuho calmly brought him to the hospital where she worked. Fortunately, his life was saved after treatment. The man gave her a beautiful ring and spoke gentle yet mysterious words. Intrigued by his strange behavior, Mizuho continued her usual routine, until one night when a giant monster appeared and attacked the city, completely shattering her ordinary life.