"The Eagle of the Ninth" is a historical novel by Rosemary Sutcliff, set in Roman Britain during the second century AD. The story follows a young Roman officer named Marcus Flavius Aquila, who embarks on a quest to recover the lost standard, or eagle, of the Ninth Legion, which mysteriously vanished in the north of Britain. As Marcus journeys through the wilds of Britain, he encounters various challenges and meets a Celtic tribesman named Esca, who becomes his ally. Together, they navigate the complexities of loyalty, identity, and cultural differences while seeking to restore honor to Marcus's family and the Roman Empire.