Archie Dare is not a troublemaker, no matter what the filing folder in the principal's office says. But it's hard to shake a peculiar reputation when living with an eccentric, bookworm grandfather whose favorite obsession is Jean Lafitte, a famous New Orleans pirate.

After attending a fancy Mardi Gras party at his one-and-only friend Lou's mansion, a break-in occurs at Archie's house, and Papa is taken. Archie receives little support from police officers and decides to investigate on his own. He is joined by Lou and his precocious, quick-witted neighbor Drea. The three discover the first of several clues Papa seemingly left behind. As Archie pieces together the first clues--a library call number that leads to a rhyme in a book about Jean Lafitte's lost treasure--he realizes the reason his grandfather was taken: he knows the location of Jean Lafitte's lost treasure.