On a winter day, Hajime Mutsuki, the leader of the idol unit Six Gravity, wanders into a secondhand bookstore and encounters Shun Shimotsuki, the leader of the sibling unit Procellarum. The two discover an antique book that appears to be the original story for a stage play they will star in, titled Rabbits Kingdom. Though the title is unfamiliar, Hajime experiences a strange, nagging sensation, as if the story is already known to him. Shun smiles gently and explains that this is a nostalgic and beloved story of themselves from another world.

The story transports them to a world of two parallel kingdoms that exist back-to-back: the Black Rabbit Kingdom and the White Rabbit Kingdom. These two halves are meant to be a pair, yet due to a twist of fate, they have never met, leaving both worlds in an incomplete and unstable state.

In the Black Rabbit Kingdom, the young king Hajime is distressed by calamities befalling the surrounding nations. His retainers, including the Prime Minister Haru and close aides Kakeru and Koi, are busy preparing for a world congress and a ball celebrating the 15th anniversary of his reign, alongside Prince Aoi and his escort Arata. At the same time, in the White Rabbit Kingdom, a person calling himself the Demon King, Shun, proposes a picnic to the Black Rabbit Kingdom. He is joined by the king Kai, Prince Rui, guards Yoh and Iku, and the former military officer now civil servant Yoru. The narrative follows what is born from the encounter between these two paired kingdoms when the White Rabbit delegation arrives for their unorthodox visit.