"Tsukiuta." Gekijouban: Rabbits Kingdom the Movie

A certain winter day. Hajime Mutsuki, the leader of the idol unit "Six Gravity," (a.k.a. Gravi) associated with Tsukino Talent Productions, aimlessly dropped by a secondhand bookstore and encountered Shun Shimotsuki, the leader of their sibling unit "Procellarum" (a.k.a. Procella).

The two of them picked up the antique classic which was thought to be the original work of the stageplay they would be starring in: "Rabbits Kingdom." Though it should have been his first time hearing the title, a strange bothersome sensation befell him, almost as if it were stuck in the back of his head.

To the perplexed Hajime, Shun gently smiled. "...Ah, this one. It's quite a nostalgic and beloved... story of us from another world." This is a "what if" story of a kingdom in some other world.

(Source: Hakusensha, translated)