In 1805, U.S. Marine Lt. O'Bannion (John Payne) and his men are sent to the African port of Tripoli to help suppress the Barbary Pirates, who've been ravaging shipping trade in the region. O'Bannion's unit is ordered to approach the pirates' base via the desert. Joining the Marines on their strategic trek is Countess D'Arneau (Maureen O'Hara), the daughter of an official. Traveling incognito, the countess soon becomes a gadfly for O'Bannion.