Detective Stephen Moran has been waiting for his chance to join Dublin’s Murder Squad when 16-year-old Holly Mackey arrives in his office with a photo of a popular boy whose body was found at a girls’ boarding school a year earlier. The photo had been posted at “The Secret Place,” the school’s anonymous gossip board, and the caption says “I KNOW WHO KILLED HIM.” Stephen joins with Detective Antoinette Conway to reopen the case --- beneath the watchful eye of Holly’s father, fellow detective Frank Mackey. With the clues leading back to Holly’s close-knit group of friends, to their rival clique, and to the tangle of relationships that bound them all to the murdered boy, the private underworld of teenage girls turns out to be more mysterious and more dangerous than the detectives imagined.