Geronimo, Trap, Pandora, Benjamin and Patti return to the era of Louis XIII on the trail of the Pirate Cats. At Fontainebleu Castle they enroll in Richelieu's army of mousketeers and are commanded by the cardinal to oppose two brigands in the area.

The mice discover that the brigands are no other than Cattardone and Tersilla who have replaced Louis XIII with Bonzo! Our heroes must find the real Louis XIII and and put him back on the throne.