Marcus searches for the Ninth Legion's lost eagle in the the treacherous lands of Britain. When Marcus was a young child, his father, part of the cursed Ninth Legion, marched into the British moors and was swallowed by the mist. He and the legion never returned. Moreover, their standard with the eagle also disappeared. A sacred symbol of pride and honor, the eagle was also feared to be a dangerous weapon in the enemy's hands.
Fast-forwarding in time, Marcus, now a young man, exemplifies a Roman soldier. Confident yet respectful, he is also bold and intelligent. He is assigned to a post in Britain as Cohort Commander. He suspects the villagers' attack, but nevertheless, when it comes the Romans are hard-pressed for reinforcements. Leading a desperate charge from the fort, Marcus is wounded, his leg mangled and cut open. Although he survives, the recovery process is long and leaves him with a lame leg.
No longer in perfect condition, Marcus is discharged from the army, after fighting only one battle in his career. He goes to live with his kind Uncle Aquila, and after restless months of inactivity and recuperation, Marcus finds several close companions.
The first is Esca, a slave captured from a British tribe. Always obedient and loyal, he is a trustworthy manservant and confidante. Marcus's second friend is Cottia, a British girl fighting against the Roman mold her family is forcing her into. Though not a tomboy, Cottia has a defiant, determined spirit. Marcus's third companion is a wolf called Cub. Marcus raised him from a pup, and Cub is a faithful pet.
Marcus's days of loitering are gone when rumors of the eagle reach him. Determined to find it, restore honor, and reform his father's legion, he and Esca, who has been freed, set off through the treacherous moors in search of it. Posing as eye doctors, they gain entrance to various tribes, but none harbor the sacred eagle.
Finally, having endured months of disappointment and misdirection, they meet an old member of the lost legion who gives them directions. They obey, make friends with the tribe, and discover the eagle. Shorn of its wings, battered and dull, it still lifts its head proudly. Marcus is overjoyed but also fearful - how is he safely going to bring it back to Roman territory?
The next day, He and Esca safely escape with the eagle, but the tribe is close behind their their heels. With the help of the former legionnaire, they cross the marsh but are still far from safety. Cleverly tricking the enemy, Marcus and Esca elude capture multiple times and finally arrive, bruised and weary, at a Roman camp.
Marcus's homecoming is bittersweet. Although he is hailed as a hero for retrieving the eagle, much to his disappointment the legion will not be reformed. And although Cub eagerly greets Marcus, Cottia, now a young lady, has changed. Thinking that he will once again leave, she is harsh with him, having missed him terribly.
However, upon receiving compensation from the Senate, Marcus decides to start a new life in Britain, rather than return to his beloved home in Italy. Together he, Esca, and Cottia look forward to a bright future.