Daisy Wells and Hazel Wong have started their own detective agency at Deepdean School for Girls, but they're struggling to find any real mysteries to solve (Lavinia's missing tie doesn't count). All that is about to change when Hazel discovers the Science Mistress lying dead in the Gym. And the mystery only deepens when the body disappears before the girls can alert anyone. Can the detective duo not only solve the murder, but prove that there was a murder in the first place - before the police get there first?