Buffy's younger sister, Dawn, knows how it feels to be different. So when she overhears her classmates teasing a new girl, Arianna, she steps in, and the two become friends. But when Buffy comes home one afternoon to find her sister and Arianna hanging out, she's surprised -- she encountered Arianna one night on patrol. A demon had been attacking her, but Arianna fought back on her own. What's that about?

The more the gang gets to know Arianna, the more puzzled they are. She has superstrength and accelerated healing power, just like a slayer. But what she also has is a dark secret and an unusual past -- and a dangerous legacy that is primed to reveal itself, just as soon as she turns sweet sixteen....