It is the beginning of December. Anna Kushina's birthday is coming up, so the boys of HOMRA decide to celebrate by preparing a surprise party for their little princess.

(Source: TubiTV)

Adolf K. Weismann is sharing a peaceful meal with his loved ones, but as the days go by, some begin to realize that this perfect world where everyone is alive and happy seems too good to be true.

(Source: TubiTV, edited)

Misaki Yata is a youth whose reckless and immature attitude has led him to be shunned by his classmates. Saruhiko Fushimi is a loner who views the world in a pessimistic way. Despite being polar opposites, the two become the best of friends.

(Source: TubiTV)

After an embarrassing high school class reunion with their successful classmates, who have "proper" jobs, relationships, and even children, the Matsuno sextuplets return home to sulk about their status as NEETs. They spend the night drinking away their sorrows, eventually falling unconscious, until they awaken in a strange world of memories, centered around one of the brothers' biggest high school regret.

As the sextuplets navigate this memory world, they encounter their embarrassing past selves from high school, as well as those of their friends. All the while, they try to uncover whose regret was great enough to trap them in this strange world, so that they can make their way back to reality. However, they're running out of time; this world of memories is unstable, and the sextuplets may lose their chance to make things right if they're too slow....