Nancy, Bess, and George can't wait to start their trip to Nantucket. The three girls are there to visit Bess's family friend Jenna and go to the opening of an exhibit at the local whaling museum. Jenna's been working on the exhibit for months, but when the girls get to the museum, a threatening banner has replaced the welcome sign.

Nancy's got plenty of potential suspects, but she's worried she won't be able to solve the case in time to save the opening. And as the threats increase in severity, she becomes more concerned about Jenna's safety than the ruined exhibit. Can Nancy find the culprit in time?