In From the Mixed-Up Files of Mrs. Basil E. Frankweiler, Claudia and Jamie sneak into the Metropolitan Museum of Art and stay there, undetected, for several nights. While they're there, a statue of an angel is on display. No one is sure who made the statue, but some people think it might have been Italian Renaissance master Michelangelo.

Claudia and Jamie decide they're going to investigate and figure out who made the statue. They start in the library, but they don't find anything there. It's only later, when they return to the museum, that they discover a clue.

In chapter six, museum workers move the statue. Claudia and Jamie go to the platform it had been on to take look. It's there that they make their discovery. They notice markings on the velvet draped over the platform where the statue had been standing. They realize that the mark must have been made by a carving on the bottom of the statue. At first, Jamie thinks he sees a W, but then Claudia makes the discovery:

"For goodness' sake, Jamie. That's not a W; that's an M." She looked at Jamie and her eyes widened, "M for Michelangelo!"

Jamie realizes he saw the same mark in a book when they were in the library. They go back to the library and find the answer: "The crushed-up mark on the dark blue velvet was Michelangelo's stonemason mark."

Claudia and Jamie are convinced they've found proof that the statue is an authentic Michelangelo. But are they correct? It isn't until the end of the book, when they pay a visit to Mrs. Frankweiler, that they find out for certain.

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