Outside the city of Cambridge, the badly decomposed body of a teenage girl is found washed up in the flooded fens. Detective Inspector Chloe Frohmann and her partner Morris Keene are called in to establish the identity of the victim. They must work quickly to solve the mystery of her death before the press pounces on the salacious story. Meanwhile, Mathilde Oliver, the autistic daughter of a Cambridge don, is attempting to trace the writer of a series of letters addressed to 'Katja', a student at Corpus Christi College who doesn't seem to exist ...