New York City girl Claudia, a mere month shy of being a twelve-year-old, has resolved to run away from home with her younger brother, Jamie. She knows that she could never pull off the classic spur-of-the-moment departure without a destination (inevitably involving having to eat outside with the insects, and cupcakes melting in the sun); so she plans everything to perfection, including their destination: the grand, elegant, beautiful, all-encompassing Metropolitan Museum of Art.

However, no sooner have Claudia and Jamie settled into their new home, than they are caught up in the mystery of an angel statue bought by the museum for the bargain price of $225. Is it in fact an as yet undiscovered work by Michelangelo, worth millions? Claudia is determined to find out, and her quest leads her to the remarkable, secretive Mrs. Frankweiler, who sold the statue to the museum - and to some equally remarkable discoveries about herself.