The story kicks off with a conversation between Iruma and Professor Balam. Balam shares some personal backstory about how he got his major injury and why he wears his mask, using it as a gentle reminder to Iruma about how dangerous the Netherworld can be. But during their chat, they realize something shocking: Iruma’s birthday already passed three weeks ago, and nobody noticed!

Because Iruma grew up with incredibly awful, neglectful human parents, he never even realized that birthdays were supposed to be important, so he never thought to mention it to his demon friends. Once the rest of the Misfit Class and his adoptive grandpa Sullivan find out, they freak out and rush to throw him a massive, spooky, yet beautifully grand demon-style birthday party called the "Ritual of the Descent of Demons.