Stephen Moran is a young, ambitious detective stuck on the Cold Case Squad, chasing after leads that go nowhere. He has his sights set on the Murder squad, but no way to get there --- until teenage Holly Mackey walks into his office with a tantalizing clue to a high-profile murder case.

A year ago, 16-year-old Chris Harper was found dead on the grounds of St. Kilda’s, a prestigious girls’ school on the outskirts of Dublin. His killer was never found. Now a photo of Chris has appeared on St. Kilda’s Secret Place, a board where the students can pin up their secrets anonymously. The caption accompanying the photo says I know who killed him.

To find the killer --- and get his chance to join the Murder squad --- Moran must work with the original detective on the case, Antoinette Conway. Conway is tough, abrasive and disliked by everyone in the department; she dislikes everyone in return, including Moran, and is still angry about how she mishandled the investigation the first time around. Both Moran and Conway need to solve this case quickly, for more reasons than just bringing the killer to justice.

As the Harper case unravels, Conway and Moran pick apart the truth from the lies deep at the heart of St. Kilda’s sprawling grounds and ancient walls. Soon they have narrowed their suspects to two separate cliques of four girls; each group hates the other, and all the girls have ties to Chris Harper. What’s worse, the list includes Holly herself, the daughter of the ruthless Detective Frank Mackey; Mackey’s been kind to Moran in the past, but he’ll do whatever it takes to protect his child. And Moran and Conway are discovering that when the secret world of teenagers spins out of control, it can be more mysterious and more dangerous than they ever imagined.