When 17-year-old Reese first moved to the wealthy, lakeshore town of St. Clair over five years ago, she never expected someone like Charlotte Walters to take her under her wing. Charlotte is everything that Reese is not: rich, gorgeous, and carefree, living with her enigmatic older brother and her parents in a mansion at the top of the lake.

But when senior year starts, everything seems off. Reese notices that Charlotte's older brother Aiden keeps coming back from college to hang around on weekends, stoic and grim. And Charlotte's parents seem more distracted, almost lost. Meanwhile, Charlotte keeps insisting to Reese that nothing is wrong, and decides to host a Halloween party with the theme of "missing girls".

That night, at the party, Charlotte goes missing.

Everyone who attended becomes an instant suspect, from Charlotte's brother (who argued with Charlotte earlier that night), to Charlotte's field hockey teammate (who wasn't invited to the party in the first place), to Reese herself (who was the last person to see Charlotte alive, out near the lake).

With the clock ticking, Reese must unravel what happened to her best friend, and dig through the layers of secrets protecting people in St. Clair. Because now, being the quiet sidekick won't cut it. And maybe only an outsider to St. Clair can truly confront all of the town's dark mysteries.