When they meet as girls on a beach in Nantucket, Maggie McIntyre and Emily Porter become fast friends — though Emily’s well-heeled mother would prefer that she associate with the upscale daughters of bankers and statesmen rather than the child of a local seamstress. But the two lively, imaginative girls nevertheless spend many golden summers together building castles in the sand, creating magical worlds of their own, and forging grand plans for their future.