Middle schooler Yuta Koizumi was born at 6:06:06 on the sixth day of the sixth month, a timing that marks him as the ideal father of a uniquely powerful demon child. That is why Lucia, a princess of the demon realm, turns up determined to win him over and have that child. Two angels, Rie Kasukabe and El Mikamoto, are posted to make sure it is never born, and since Rie is also the classmate Yuta likes, his ordinary days fill up with the supernatural.