Has the world suddenly split into four? Keiwa Sakurai, Neon Kurama, Michinaga Azuma, and Tsumuri are flung into different worlds. And for some reason, a different Ace exists in each of the four worlds. The cause seems to be from the "World Doomsday Game" started by the staff from the future. In light of this crisis, the mysterious Cross Geats appears. Just what will happen to Ace and his friends?

Cherry blossoms are in full bloom. It is time for college students to go on a drinking spree under the cherry tree - It's "Hanami" time! Two freshmen and a sophomore are ordered to stake out a spot for the entire pep squad. Plans are initiated from the night before; three of them have to pull an all-nighter. Once the night falls, the sky turns pitch black. Rival pep squad of the Roka College emerge from the darkness and an all-out turf war begins.

Wracked with guilt over her younger sister's suicide, pretty young karate student Megumi accompanies a group of older friends on a camping trip into the woods, where they're attacked by feces covered undead.

The story takes place after the TV drama series Ossan's Love (2018). It's been a year since Haruta and Maki pledged their undying love. Haruta has now come back and receives a warm welcome from familiar faces, his boss Musashi Kurosawa and other colleagues. Justice Yamada, a recruit who has been recently assigned to this branch, welcomes him too. One day, completely out of the blue, the headquarter-based project team "Genius 7" shows up at the Second Sales Branch. Project leader Jin Mamiana orders the Sales team to support them in their big project. Maki, who had been transferred and works now at the head office, is among the project team members with Mamiana. Haruta and Maki grow apart, and Haruta teams up with Justice. Then Kurosawa gets involved in an accident and loses his memory, but the loss of his memory is selective and he has forgotten only Haruta. However, when Kurosawa sees Haruta, he falls in love again.. Hilarious and heartwarming, the final battle of Ossan's Love begins.