In a certain kingdom, a prophecy was made about a child born with a birthmark on his back. It was said that he would bring an end to the kingdom, and that his death would bring about its destruction. When the twin prince and princess were born, Len and Rin, Len was discovered to have the birthmark. He was banished to a lonely tower, and Rin was raised as the only princess. However, it ended up being impossible to keep the siblings apart, and soon they formed a deep friendship. Although circumstances may pull them apart, nothing can break the bond they have with each other. They struggle together to survive the treacherous and dangerous world they were born into. This is their story.

Based on the VOCALOID songs Daughter of Evil and Servant of Evil.