When eleven-year-old Jane Palmer moves to Falmouth, Massachusetts on Cape Cod for the summer, she and her siblings are looking forward to reading books, going to the arcade, and walking to the beach from the family summer home. Before long, Jane's nine cousins and their families unexpectedly arrive to stay for the summer. While playing Super Secret Spy Club, the thirteen kids discover that their grandparents are planning to sell the family Heights House because it is too expensive to keep it up. Heartbroken, the cousins set out to save the home that has been in their family for generations. Glimpsing ghostly visitors and reliving faded memories, the thirteen cousins hear tell of a long-lost Palmer family treasure, but it is up to all of them to find it before the Heights House is sold for good.