Karenai Sekai to Owaru Hana

In a future over one hundred years hence...

All the people in the world have awakened to unique abilities called 'Dunamis'. As these special powers become rooted in people's everyday lives, their value system changes, and those who lack powers are the ones who are treated as strange. With the passing of the ages, the environment has changed completely, while society has greatly regressed. The overall population of mankind has diminished as well, and most of the world is made up of wastelands and greenery dotted with the weathered ruins of the world before.

The City of Raillore.

With brickwork buildings like those of the Middle Ages, developed over land graced with lush greenery and rivers. With the concept of nations gone from the world, Raillore exists as something of a self-sufficient commune. The Order of Security and Regulatory Knights - AKA the Regulatory Corps - serves as a government organization in charge of handling crime prevention and other tasks that require force of arms.

But things begin to change as odd incidents take place, one after another...

Those possessing abilities different from those of Dunamis begin to appear. The people fear this power and the phenomena it causes, and those capable of it are designated as ‘Errors’. The sudden appearance of these Errors is regarded as a disease, and the handling of Errors is placed under the jurisdiction of the Regulatory Corps. As people tremble in fear of the vicissitudes of the changing times, there are those who try to open a way to the future with their own strength...
 

Uchihama Academy has seen an increase in enrollment in recent years and as a result, a new school building will be constructed. The story takes place during the final school festival in the old school. All the club leaders decide to give it their all to make this the best school festival. Sou is a member of the notorious astronomy club, which basically solves the various issues in the student council's place.

On one day, he stays in the school late at evening because he got slightly injured by the judo club and is healing his wounds in the infirmary. Suddenly, the building shakes. Shocked, he runs up the stairs to where the shaking seems to have centered, and there he encounters a mysterious naked girl, who at first appears to know him. On the next day this girl, Yui, seeks out the astronomy club, expressing her desire to join.

Having recognized Minazuki Kashou’s talents in running his patissiere, Beignet felt her role was over. So she closed La Soleil in France and entrusted Fraise, the catgirl she raised, to him.

Fraise feels a fondness towards Kashou but becomes bewildered in the face of his 6 catpanions. She turns to Shigure for advice but she too harbors a hidden love for him. Fraise feels that the relationship between siblings is the most important while Shigure feels the happiness of her catgirls is what is most important.

Motivated by their desire to make sure the other is able to fulfill their love for Kashou first, the battle between catgirl and girl begins!

At the Saint Michael Girls' School... festivities erupted among the girls of the annex. The reason for their celebration is the Best Couples poll being held at the main campus. The students at the annex, who look up to the girls from the main campus, decided, "Let's have our own Best Couples!"
...It was always inevitable.
And so, the fairest and most intimate couples were nominated...

The first couple is a pair of childhood friends, Misawa Nagisa and Takahata Rina. The second couple is a pair of twin sisters, Kimishima Ai and her younger sister Aya. And finally... The transfer student who saw this whole thing as no concern of hers, Onohara Hazuki, is shocked to hear that she too, was chosen to be among the Best Couples.

Moreover, her partner is the sheltered young debutante, Suoh Manami. Actually, Hazuki has secretly been crushing on the impeccable Manami... But the truth is, they're far from a couple, and while their classmates may cheer them on, it's all in vain.

But then... Hazuki works up all her courage to ask Manami to become a "Best Couple" with her, and...