From The Mixed-Up Files of Mrs. Basil E. Frankweiler

Pre-teen Claudia, adolescent Jamie, and infant Kevin are the three children of the Kincaids of Madison, New Jersey. Claudia is prone to flights of fancy - especially wishing that she lived the life of Lady Guinevere - out of which her father hopes she will grow. Claudia senses an opportunity when she finds in the trash an unused train pass, good for her and one other child. Needing Jamie's money - $24.40 which he obtained from years of gambling with a friend - but also wanting his company, Claudia convinces him to run away with her using that train pass. With Jamie controlling the money, Claudia does not divulge her complete plan to him until it happens: they will take the train into New York City and live as hideaways in the Metropolitan Museum of Art, where they can indulge in fantasies of others' lives and worlds through the museum's many exhibits. Their primary tasks of getting basic necessities while evading the museum's security guards change when Claudia spots a sculpture of an angel in a work room, and learns the story that it was sold to the museum by a Mrs. Basil E. Frankweiler for a pittance of its probable worth, especially if the rumors that it is an authentic Michelangelo are true. To validate the reason she decided to run away, Claudia, with Jamie by her side, has to find out the truth behind the sculpture, and believes that reclusive Mrs. Frankweiler has the answers.