Seventeen-year-old Fenny Allen has been Sarah Vincenty's sidekick for as long as she can remember. Over the years, Sarah has built a reputation in their small town as an amateur detective and a vigilante who dispenses her own form of justice.

Now Sarah is missing. The police have written her off as yet another teen runaway, but Fenny is convinced there’s foul play. And when she discovers a binder Sarah left behind, entitled In Case I Go Missing, Fenny realizes that Sarah disappeared while investigating her biggest case yet. 

With Sarah gone, it's up to Fenny and her other best friend, Sam, to finish her investigation. As they piece together the clues, they find themselves up against a decades-old mystery that links to the town's dark history. But is this binder a guide to finding Sarah—or are Fenny and Sam already too late?

The more the pair uncover, the clearer it becomes that those involved will go to great lengths to keep their secrets. And when everyone has something to hide, there are more than a few reasons to want Fenny and Sam out of the picture. Permanently.