Everyone always knows something, even if it's something they don't know they know. Very unpleasant things happen at a girls' school... In the middle of the night, while the whole school is asleep, two teachers, searching for the source of a mysterious glow in the Sports Pavilion, discover the body of the gym teacher - she has been shot at close range. The school plunges into chaos, however, when the "cat" lurking in the pigeons' nest strikes again. Unfortunately, one student knows too much. And more specifically, she knows that without the help of Hercule Poirot, she will be the next victim...