Sealed with a secret

In the beautiful Cartier makeup compact she bought at a flea market, Phoebe finds a letter written during World War II in London, from one sister to another, which contains a "spell" for bringing people closer together, a kind of treasure hunt around London--and Phoebe could really use such a spell, because her older sister, Alice, will be leaving for university soon, and just now she seems to be cutting Phoebe out of her life.