Harriet is awakened in the middle of the night by Annie and her fellow servant, Carrie, who claim to have caught the vandal in the Science-Lecture room. They’ve already notified the Dean, and she has now summoned Harriet. When she arrives at the scene, Harriet finds that the room has been locked from the inside, and the vandal made her escape through a rear window after knocking over a blackboard. Annie says she caught a glimpse of someone in an academic gown seated at a table, but her back was to the window. By the time Carrie looked through the window, the culprit was gone. Upon inspecting the interior of the room, Harriet notices newspaper, cutout letters, cardboard, and glue on the table. The Poison-Pen had been at work just before she took flight.

After dawn, Harriet calls Wimsey and asks him to stop by later in the morning rather than wait until six o’clock in the evening. He arrives half an hour later with his manservant, Bunter, a camera, and a fingerprint kit. Wimsey goes over the Science-Lecture room methodically in search of clues. He finds two black hairpins—one by the table and one by the escape window.