Daisy and Hazel are finally back at Deepdean, and the school is preparing for a most exciting event: the fiftieth Anniversary.

Plans for a weekend of celebrations are in full swing. But all is not well, for in the detectives' long absence, Deepdean has changed. Daisy has lost her crown to a fascinating new girl - and many of the Detective Society's old allies are now their sworn enemies.

Then the girls witness a shocking incident in the woods close by - a crime that they're sure is linked to the Anniversary. As parents descend upon Deepdean, decades-old grudges, rivalries and secrets begin to surface, and soon Deepdean's future is at stake.