The opening of the book introduces us to Lucile, who awakens on a stormy night to a mysterious tapping sound on the yacht, raising her anxiety about the situation. As she contemplates the strange noises, we learn about the background of the girls: their recent return from adventures in Alaska and their modest circumstances forcing them to live on the O Moo instead of in university accommodations. The tapping ultimately leads to the discovery of a cryptic note urging them to leave, suggesting that they have caught the attention of someone with ill intent. The suspense builds as various elements are introduced, including Lucile's determination to uncover the source of the tapping and the mysteries surrounding their living situation. (This is an automatically generated summary.)