Marcus is the new commander of a Roman fort on the northern frontier of Britannia. He has chosen this outpost to follow in his father's footsteps. His father was the commander of the first cohort of the Ninth Legion that disappeared in the northern wilds and was never heard from again. When he arrives at camp, the prior commander has already left. He wanted to leave as soon as he was able. The men are under supplied. Marcus wants to send out a patrol to investigate the grain shipments. The other officers feel it is too dangerous, but they are well fed. Marcus senses the officers have gotten too conservative given their relatively comfortable situation. He sends out the patrol anyway.
He awakes in his sleep that very night. Strange rustling can be heard in the treeline. He wakes up the men and has them marshal for a potential attack. The other officers think he is just hearing things. The Britons attack.
Due to preparedness of the men, they repel the attackers. The next day, the Britons return with the captured patrol in bonds. They execute a member of the patrol as the Roman soldier's in the fort watch in horror.
Marcus leads a detail out of the fort to rescue the men. They use their shields and military training to drive a wedge through the Britons and form a defensive circle around the surviving patrol members. The Britons bring chariots and crash the circle severely wounding Marcus, but the patrolmen are saved and this second attack repelled.
Marcus's bravery earns him a medal. His wounds earn him a discharge. He feels disgraced despite the commendation. A better surgeon tends his wound and he is no longer a cripple.
With the help of Esca, a slave he saved from the gladiatorial arena, he sets north to find the Eagle Standard of the Ninth Legion and regain his family honor.