Casey, a young girl living in a foster home, is recruited by Harry to impersonate Margaret, the long-lost granddaughter of Lady St. Edmund, the owner of Candleshoe estate. Harry believes that the treasure hidden within Candleshoe is the key to his redemption and convinces Casey to help him find it.
As Casey settles into life at Candleshoe and gets to know Lady St. Edmund and the other residents of the estate, she begins to feel guilty about deceiving them. She eventually decides to come clean and tell Lady St. Edmund the truth about her identity.
Meanwhile, Harry's true motives become more apparent as he becomes more desperate to find the treasure. He begins to search the estate more aggressively, causing chaos and suspicion among the residents.
Casey, with the help of Lady St. Edmund's grandchildren and the estate's caretaker, sets out to follow the clues left behind by Lady St. Edmund's late husband, in order to find the treasure before Harry does. Along the way, Casey learns the importance of honesty, family, and loyalty.
In the end, Casey and her new friends are able to uncover the treasure and save Candleshoe from Harry's destructive plans. Lady St. Edmund is reunited with her long-lost granddaughter, and Casey finds a sense of belonging and family at Candleshoe.