When Claudia decides to run away from home, she plans the perfect escape. Because she needs money, she invites her penny-pinching younger brother Jamie to come along. And because she enjoys living comfortably, she chooses the elegant Metropolitan Museum of Art in New York City as their hideaway. While camping out in the treasure-filled rooms of the museum, they'll dodge night watchmen, sleep in a royal bed, and take baths in the public fountain.

Claudia and Jamie discover something new every day, but the most fascinating thing of all is a small marble statue named Angel. It was bought at an auction for only $225, but some think it may have been carved by the Renaissance master Michelangelo, and could be worth millions! Only the original owner, Mrs. Basil E. Frankweiler, knows the truth. Somewhere in her mixed-up files are the answers to all of Claudia and Jamie's questions—not just about Angel, but about themselves.