From here:
Sunao always thought she'd fall in love when she started junior high school. But there aren't any attractive boys at school. Still Sunao doesn't give up and meets two handsome upperclassmen. The kind and sensitive Toru and the cheerful and friendly Kaoru are members of the school Magic Club, which Sunao also joins. Then she finds out that Toru and Kaoru are actually thieves known as "B-apostrophe" who need Sunao's help as a master of the martial art Shinbu-ryu. They are trying to find the treasures hidden by the illustrious thief from the Taisho Era.

According to the legend, whoever finds seven glass balls and solves the seven secrets, hidden in each, will win treasures worth 30 billion yen and learn a strange magic secret. Sunao is surprised by this and becomes part of the team because she is attracted to the boys. It's like being in heaven surrounded by two such beauties.
But they are not the only ones after the treasures. Somebody called "B" always gets in their way. "B-apostrophe" might know who "B" is... Sunao is swayed by her complex feelings: She likes Toru, but occasionally feels a pull towards Kaoru and even to "B" when he makes contact with her!