From ShoujoMagic:

Iizuka Sono just transferred to a new school where the reigning oddballs have their own club called "BxB Brothers" ("B" for "Baka" = "Idiot"). According to them, you make your life meaningful by doing things out of the ordinary. While Kazuhisa and Kei aren't too selective about membership, Katsumi is -- so it's just the three of them... Until Sono strikes Katsumi as an admirably unusual person, and he coerces her into becoming a Brother!

The more Sono learns about the Brothership and Katsumi, the more pride she takes in being a Brother, but being the only BxB Sister has its problems. Caught between the open affection of Kazuhisa and the reserved admiration of Katsumi (with Kei taking school-wide bets on the side), all while struggling to register the club officially, it's a BxB Triangle that constantly takes on new angles!