Brother Saves the World.Late on night, Shüsuke spots a sexy cosplayer named Ran Yatagai on top of a vending machine. He later sees Ran, who calls herself Princess Leila, at school the next day, who invites him and his friends to the rooftop. She claims to be seeking a saviour amongst the scum of the Earth in order to defend it, with Shüsuke and friends believe to be initiation for some kind of underground club. As Nao and Iroha decide to follow her, they meet her twin sister, Rin, who reveals Ran is using her own peculiar method to find a partner...