During an interview, Mizuki Yuu was hit by a truck and flew away. When he woke up, he found that he had traveled to a parallel world and turned into a child. He was picked up by a plump, mature and sexy older sister and taken in.

He thought he could live the comfortable life he had always dreamed of, but he found that this world seemed a little wrong!

The mature older sister who took him in was called Mizuki Shiranui, a Taiman Ninja. Shiranui's daughter, his non-blood-related sister, was called Mizuki Yukikaze. The new wife next door, Mrs. Shendong, was a snow girl. Did a classmate who had a good relationship with him suddenly become a succubus?

Someone in the school was holding a mobile phone and was ready to hypnotize all the girls with a software. The female knight of the demon world tried to seduce them to join the demon clan. In this somewhat broken world, Mizuki Yuu just wanted to protect the people around him and keep them from the covetousness of various yellow-haired people.

But they asked why they all relied on themselves one by one?