Tomboy Casey Brown leads five boys running through downtown Los Angeles, California, stealing a basketball, spilling a drum of used motor oil, knocking over crates at a fruit market and shoplifting. When Casey gets home to her foster parents’ apartment, a man named Jenkins is waiting. Confirming the presence of a scar on Casey’s right shoulder, Jenkins gives her foster father an envelope filled with cash and leads her away to what Casey believes is juvenile hall. She is surprised when Jenkins instead takes her to a modern hotel, checks her into a room and directs her to take a bath. After Casey is scrubbed and garbed in a new dress, Harry Bundage arrives. He looks her over, asks where she got the scar on her shoulder and slaps her face when she gives him a sarcastic reply. Casey says she doesn’t remember, and Harry tells her to “keep remembering not remembering.” He gives Jenkins a check and hurries Casey out of the room. Later, at a restaurant, Harry tells Casey he’s an investor, but Casey insists he is a con man. Harry lays out his plan: An American adventurer had a child named Margaret with a British countess, and when Margaret was three, her father kidnapped her to America. The mother died soon afterward, and the father was killed a year later in an automobile accident. Margaret’s fate is unknown, but like Casey she would be a fourteen-year-old orphan with a scar on the back of her right shoulder. Harry wants to present Casey as Margaret, the fourth marchioness of St. Edmund, to her grandmother, Lady St. Edmund, at Candleshoe Manor in England. Casey agrees to the deal but cuts her own terms with Harry: ten percent of whatever they get, plus a red Ferrari. Harry says they’d better get out of Los Angeles quickly before Jenkins tries to cash his check. In London, England, Casey meets Harry's cousin, Clara Grimsworthy, a former servant at Candleshoe who knows many St. Edmund family secrets. Clara’s job is to coach Casey on Margaret’s likes and dislikes—allergic to strawberries, loves rice pudding—and other particulars, including the names of Margaret’s stuffed animals and the melody, “Greensleeves,” that plays on a music box that Clara hid behind a brick in the main fireplace at Candleshoe. Casey’s job is to hunt for a series of clues to hidden booty that was left somewhere on the estate by eighteenth-century buccaneer Capt. Joshua St. Edmund. The first clue comes from Joshua’s will, which Clara found inside a bedpost at Candleshoe: “For the sunrise student there is treasure among books.” Under the will’s wax seal is a Spanish gold doubloon taken during one of Joshua’s piracies. When Harry writes to Lady Edmund to say he has found her granddaughter, the countess answers back that even though she has been through these interviews before, she is nonetheless willing to see the girl. As Harry and Casey arrive at the manor, they see several children on the roof. Candleshoe’s butler, Priory, leads the two into the main room where Lady St. Edmund is waiting. Casey gives a convincing performance, turns down an offer of strawberries, pretends to like rice pudding, and keeps staring at the fireplace because it stirs memories of a fireplace that had a hole in it. Then, as a bluff, Casey tells the countess that she’s just acting, she’s not really Margaret and she wants to leave. Harry is shocked by Casey’s confession, but he can’t do anything except play along. After he and the girl leave the room, Lady St. Edmund tells Priory to check the fireplace. He finds a loose brick and a small music box behind it. The countess calls Casey back into the room and gives her the music box. Casey studies it for a moment, begins to hum “Greensleeves” and then opens the lid. The box plays “Greensleeves.” Lady St. Edmund is convinced: Casey is her granddaughter, Margaret, the fourth marchioness of St. Edmund. She introduces Casey to four children—Cluny, Peter, Anna and Bobby—she has rescued from a local orphanage. That evening, the countess comes to Casey’s new bedroom and places two stuffed animals on her pillow. Casey greets them by name: Teddy and Piggywig. The next morning, as Casey explores the library for clues “among books,” Cluny, a girl roughly Casey’s age, comes to get her for breakfast. At the table, the kids have an easy relationship with Priory. When Lady St. Edmund comes into the room to invite Casey on an introductory stroll around the estate, she asks Priory why morning roses aren’t on the table. The children assist Priory as he disguises himself as “Gipping” the gardener and runs to the garden in time for Lady St. Edmund to come by and scold him. Late that night, as Casey searches the library, one of the bookshelves opens and Priory emerges from a secret passage. He tells Casey that the house is full of shortcut passages and he is there to find a book on plumbing. Priory confides that the St. Edmund family is nearly penniless and acts in the role of many different servants—a ruse he is able to pull off because the Lady St. Edmund does not wear her glasses. He and the children keep the manor going by giving tours and selling produce from the garden at a street market in the village. The next morning, when Casey returns to the library, the rising sun projects a message written on a stained glass window onto the opposite wall: “The paths of glory lead but to the grave.” When she asks Lady St. Edmund about it, the countess says it’s a line from Thomas Gray’s eighteenth-century poem, “Elegy in a Country Church-Yard.” As Casey furtively calls Harry to pass along the clue, Cluny comes into the room and tells Casey that she knows she’s up to something. Later that day, after Priory pretends to be "John Henry," the chauffeur, and takes Lady St. Edmund into town, Cluny and Casey get into a fight that ends with both of them in a pond. Later, when Lady St. Edmund asks how Casey got a cut on her face, she lies to protect Cluny. Pursuing her newest clue, Casey goes to the nearby graveyard that night and runs into Harry and Clara. Not far from Joshua’s crypt they find the tombstone of a seaman named Thomas Gray whose epitaph says that he “followed the eclipse for riches.” Harry tells Casey that an eclipse of the sun or moon must be the next clue. The next day, Casey accompanies the children to the village market and uses her salesmanship to raise more than the one hundred pounds that the family needs to pay taxes, but when she returns to Candleshoe, Bundage steals the money. As Casey tries to stop him from driving away, she is thrown against a tree and knocked out. The children visit Casey at the hospital to report that without money for taxes, everything at the manor is being auctioned off; they will have to return to the orphanage and the countess will go to a home for elderly people. Casey rushes home and finds Priory waltzing with Lady St. Edmund. The countess confesses that she knew along that Priory was playing several roles for her benefit, and Casey admits that she came to Candleshoe to find clues to a fortune somewhere in the house. When Casey mentions the word “eclipse,” the countess says that Joshua’s pirate ship was called Eclipse. There was an oil painting of the ship in the house, but it was sold that morning. The person who brokered the sale tells Priory the painting is on its way to London by train. The whole family jumps into the car and chases the train until Priory stops in the middle of the tracks and forces the engine to stop. When the conductor lets them into the baggage car, they see that Joshua is portrayed in the painting holding an upside-down scroll that says the treasure is “underfoot in the great hall.” They rush back to Candleshoe and find Harry, Clara and several henchmen tearing up the hall. A struggle ensues; Harry and Priory fight each other with battleaxes and swords as the children play hit and run with Harry’s minions on the slippery floors. When Harry hits a post with an axe, he knocks down a balcony holding a bronze statue of Joshua standing with one foot on a treasure chest. Gold doubloons tumble from the broken chest as police arrive to arrest the trespassers. Ashamed that she has betrayed the family she has come to love, Casey slips away to the train station in hopes of getting back to Los Angeles, but Lady St. Edmund brings her back to Candleshoe.