After saving a strange fuzzy demon, Hanae Ashiya ends up spending his first week of high school in the nurse's office; the demon latched onto him and started absorbing his energy, deteriorating Ashiya's health. One day, he accidentally finds a flyer related to exorcism and desperately contacts the exorcist Haruitsuki Abeno for help. They meet in the Mononokean—a suspicious-looking tearoom that Abeno owns—and though reluctant to exorcise the creature at first, Abeno helps Ashiya before asking for a fee. Unable to pay, Ashiya has no choice but to work for the Mononokean to pay off his debts. As time passes, the two boys learn to work together and their perspectives start to change, while secrets are revealed, one by one.

Fukigen na Mononokean has been simulpub in English as The Morose Mononokean by Crunchyroll since July 5, 2015.