Mona Pringle, the local 911 operator, is calling Charlotte Adams with her own emergency: Serena Redding, a high school “mean girl” who used to torment Mona, is coming back for a reunion. When Mona talks about how good it would feel to kill Serena, Charlotte doesn’t believe she means it. But when a woman who looks like Serena is killed in a hit-and-run, and another former mean girl is also run down, Charlotte realizes she needs to look both ways for the now-missing Mona.