For some reason, Sasami decides to seclude herself in her room over spring break, using her power to create a barrier that seals off her home from the outside world. Feeling this will have implications on the world if it goes on, Kagami, Tama and Kamiomi attempt to break their way inside the barrier, where they are met by Sasami's resistance. After struggling a bit, they are soon joined by Tsurugi, returned from the underworld, who instructs Tama to eat her way into the barrier and bust into Sasami's bedroom. There, they discover the reason for Sasami's seclusion: she had suddenly gotten fat overnight. To explain what happened, Tsurugi shows Sasami the past through the perspective of Juju, learning of a side of her that she never showed to Sasami. Just then, an assassin from the evil society Arahabaki named Tamamo-no-Mae appears with a stone golem, capturing the young Sasami and speaking of changing the future.
『天岩戸』を出現させ、“引きこもり”ならぬ“立てこもり”を始めたささみさん。心配したかがみ、たま、お兄ちゃんは強行突破を試みるも、最高神のちからで迎撃するささみさんに太刀打ちできない。ようやく根の国から帰還したつるぎの協力もあって、天岩戸を突破したかがみ達だったが、ささみさんが立てこもっていたのには、誰にも相談できない理由があって…。
Sasami s'est retranchée dans sa chambre et ne veut voir personne. Même son frère est interdit d'entrée. C'est sans compter sur Kagami qui a bien l'intention de venir en aide à son amie pour résoudre ses problèmes...
