Alex Morningside is a 10 1/2 year old girl who's often mistaken for a boy; she wears her hair short and is something of a tomboy. Orphaned at a young age, she lives with her uncle in their home above his doorknob shop. When Mr. Underwood, a new teacher, shows up in her sixth grade classroom at the prestigious Wigpowder-Steele Academy, Alex finds herself finally liking school. Mr. Underwood has a good sense of humor and is fun to talk with.
Mr. Underwood also has a family secret - he's the descendant of a famous pirate family. When he's kidnapped by a rival pirate family over a long-secret buried treasure, Alex is the only one who can help him. She goes on a journey that will take her through strange places, where she meets equally strange people and one Extremely Ginormous Octopus.