Fifteen-year-old Aki Simon has a theory. And it's mostly about sex. 
No, it isn't that kind of theory. Aki already knows she's bisexual—even if, until now, it's mostly been in the hypothetical sense. Aki has dated only guys so far, and her best friend, Lori, is the only person who knows she likes girls, too. 
Actually, Aki's theory is that she's got only one shot at living an interesting life—and that means she's got to stop sitting around and thinking so much. It's time for her to actually do something. Or at least try. 
So when Aki and Lori set off on a church youth-group trip to a small Mexican town for the summer and Aki meets Christa—slightly older, far more experienced—it seems her theory is prime for the testing. 
But it's not going to be easy. For one thing, how exactly do two girls have sex, anyway? And more important, how can you tell if you're in love? It's going to be a summer of testing theories—and the result may just be love.