From the mixed-up files of Mrs. Basil E. Frankweiler

Claudia is tired of being the oldest and the only girl, so she plans her running away down to the last detail, including bringing her little brother Jamie along because he has saved every penny he's ever gotten. They hide out in the Metropolitan Museum of Art, sleeping in an antique bed and bathing in the fountain, until Claudia becomes fixed on a small marble statue nobody can explain.