What's wrong with Mary Robinson? That's what Jessica and Elizabeth Wakefield want to know. She has a rich new stepfather who has a fabulous yacht and is going to build them a huge house. So why has Mary been acting so strange?

Jessica is confident Mary will be herself again, just as soon as she finds out about the surprise birthday party for her on her father's yacht! But Jessica is wrong - a party on the yacht is the last thing Mary wants. It could give away her whole secret. In fact, all of Mary's little white lies would get her into big trouble with her friends! Will anyone want to help poor Mary when they learn the terrible truth?