Many people have a book that they read over and over again as a child. This one was mine. It begins with the Drew children indignant that the grail, the artifact they discovered in Over Sea and Under Stone, has been snatched from the museum, presumably by the powers of the Dark. They return to Tresswick to try and solve the mystery, and encounter none other than Will Stanton, who is there on the same errand. Having the characters from the first two books of the series meet up lends the story the delightful excitement of getting some of your good friends that don't know each other together for the first time.
My favorite part, though, was the Greenwitch. A creature of the wild magic drawn from an ancient spring rite, the Greenwitch holds beneath the sea the secret key to the grail. Without this key, the grail is useless to either Dark or Light. Who will risk drawing her forth and unleashing her untameable powers? Who can persuade her to give up her precious treasure?
Oh, you'll never guess! You'll just have to read it.