Feeling underappreciated by her parents and the world in general, twelve-year-old Claudia Kincaid decides that the best way to teach them a lesson is to run away from home for a while. But the only thing Claudia hates more than being underappreciated is being uncomfortable, so she chooses to run away to the Metropolitan Museum of Art in New York City (for all those poofy Marie Antoinette beds to sleep in), dragging along her nine-year-old Card Sharp brother Jamie, whose impressive life savings of $24.43note ensures that they can afford to hide out in style.
Thus begins the adventure of a lifetime for both of them. Between hiding from museum security in the bathroom in the mornings, tagging along with tour groups all day, and bathing in the museum fountain by night, Claudia becomes obsessed with a mysterious marble angel, sold to the museum for a mere $225, that may or may not be a lost Michelangelo.
And why is eccentric multi-millionaire Mrs. Basil E. Frankweiler writing such a long letter to her dear lawyer Saxonberg?
This story provides examples of:
- Big Fancy House: The Frankweiler mansion. Claudia is particularly impressed by the bathtub.
- Catchphrase: Jamie's is "Oh, boloney!" Turns out to be helpful for Claudia to remember the city of Bologna, Italy.
- The Confidant: What Claudia becomes for Mrs. Frankweiler, to both their benefits.
- Cool Old Lady: Mrs. Frankweiler, who is old and rich and amuses herself by setting the Metropolitan Museum of Art on a wild goose chase as to who the sculptor of the angel is, simply so she can watch the fun from afar. Then she gives the invaluable secret of the statue to a twelve-year-old girl she's only met once, solely on the basis of recognizing a kindred spirit.
- Death by Newbery Medal: Refreshingly subverted. Everyone in the book not only survives but is never even imperiled. The closest we get is Mrs. Frankweiler herself, who is elderly and in the process of writing her will... but she's feisty enough that one gets the feeling she'll survive at least another decade or two.
- Distracted by the Luxury: Claudia is so distracted by the opportunity to bathe in Mrs. Frankweiler's luxurious sunken black marble tub that she almost forgets why they came there (giving Jamie an opportunity to spill the beans).
- Eat the Evidence: On the night before they leave, Claudia leaves a list of instructions for Jamie, which he is to destroy after reading. He decides to do so by eating it. On taking a couple of bites and finding that it tastes like dried-up bubble gum and turns his teeth blue, he tears up the note and throws it in the trash.
- Eccentric Millionaire: Mrs. Frankweiler, who sells a genuine Michelangelo to the Met for pocket change, then refuses to provide authentication just so that she can watch the curators squirm.
- Epistolary Novel: The entire book is a single long letter from Mrs. Frankweiler to her lawyer, Saxonberg.
- "Eureka!" Moment: Jamie's Catchphrase reminds Claudia that Bologna, Italy, had been mentioned in connection with the statue in an article she'd read.
- Fancy Camping: Claudia's reasoning for choosing the Met is that she wants to run away to somewhere clean, comfortable, and beautiful.
- Free-Range Children: Police and journalists' reaction for missing children is really muted in the 1960s compared to today.
- Grammar Nazi: Claudia has a bit of a habit of correcting her brother's grammar.
- Gratuitous French: Mrs. Frankweiler offers Claudia and Jamie a lunch of nouilles et fromage en casserole... and laughs when they realize it's just macaroni and cheese.
- Long Title: From the Mixed-up Files of Mrs. Basil E. Frankweiler. E. L. Konigsburg is fond of such titles in general.
- Narrator: Mrs. Frankweiler narrates the story in her letter to Saxonberg.
- Non-Residential Residence: The protagonists live in the Metropolitan Museum in New York for most of the book, hiding and dodging guards.
- One Degree of Separation: Saxonberg, Mrs. Frankweiler's long-time lawyer, is also Claudia and Jamie's grandfather.
- Public Bathhouse Scene: An unusual example in that it's not in an actual bathhouse, but rather in the Met's fountain after hours. There's even a double-page spread of Claudia and Jamie nude in that fountain! note
- The Runaway: Claudia and Jamie.
- Saying Too Much: Jamie accidentally reveals where they've been hiding out to Mrs. Frankweiler while Claudia's upstairs in the bath. His confession skims very close to I'll Never Tell You What I'm Telling You!.
- Senseless Violins: Claudia and Jamie pack most of their clothes in Claudia's violin case and Jamie's trumpet case.
- Swiss-Cheese Security: The Met overnight security.
- The Unfavorite: Claudia's sick of being the oldest kid in the family, with all of the responsibility and none of the appreciation. She just wants to feel special.
- Unusual Euphemism: Jamie's favorite minced oath is "boloney", which gives Claudia the "Eureka!" Moment required to solve the Michelangelo mystery.