From the Mixed-Up Files of Mrs. Basil E. Frankweiler takes the form of a letter from Mrs. Basil E. Frankweiler to her lawyer, Saxonberg. And what a letter it is.
Twelve-year-old Claudia Kinkaid decides to run away from home with the help of her little brother, Jamie (who is just nine years old). With Jamie's money and Claudia's smarts, they bust out of that suburban joint and run to somewhere that any kid would love. Disneyland? The rainforest? Sweden? Nope—they end up at the Metropolitan Museum of Art. Less kid-friendly, but more filled with pretty, elegant things.
They reach the Met and fall into quite the routine: they wander around and join tour groups during the day, hide in weird exhibits at night, and try to spend as little money as possible… well, at least Jamie tries to keep the moolah locked down. Their adventure gets way more adventuresome though, when they discover the hubbub around a recent museum acquisition, a marble statue of an angel that may or may not be carved by Michelangelo. You may not know this, but Michelangelo is sort of a big deal, at least in the art world. Obviously, a twelve-year-old and a nine-year-old are going to solve a big mystery in the art world that all these historians and museum officials are stumped by. No big deal.
All their time spent in the library and sleuthing around the museum can only get them so far, so they track down the person who sold the mysterious statue to the museum in the first place—one Mrs. Basil E. Frankweiler (sound familiar?). When they get to her big old house, they ask Mrs. Frankweiler about the angel statue and she agrees to let them know what's up on two conditions: (1) they must find the file on the angel statue in her filing room in under one hour, and (2) they must tell her all the details of how they ran away.
What a crazy old lady—willing to part with that much money for some kids' explanation of why they ran away. Never mind… the kids are up to the challenge. They go through all those mixed-up files (without mixing them up more) and find out that it's true. It's true! The angel statue was made by the great Ninja Turtle… we mean, Renaissance artist Michelangelo!
Mrs. Frankweiler says that she'll give them the files on the angel statue in her will, but that they can't tell anyone in the meantime before she dies. Oh well. Claudia wouldn't want to let anyone else in on her secret anyway.
The kids can go home feeling like they've done something special, and after the kids have left, both they and Mrs. Frankweiler contemplate whether or not they'll visit each other in the future.
Oh, and it's also revealed in the end that Saxonberg, Mrs. Frankweiler's lawyer, is Claudia and Jamie's grandfather. Small world.