The film begins with legates Quintus Titurius Sabinus and Lucius Arcunculeius Cotta, the commanders of one of the winter camps in Gaul where a legion of Roman soldiers are stationed. The camp has been under heavy siege by Gallic tribes. They are arguing about an invitation from Ambiorix, head of the coalition of Gallic tribes attacking, to parley. Sabinus wants to meet Ambiorix to try and be allowed to retreat without being harmed. Cotta wants to stay at the camp and try and hold off the Gauls until help can arrive. Sabinus thinks that is suicide and having seniority goes with the option of going to parley with Ambiorix. Taking a small garrison with them they meet with the Gallic chieftain. By the time they realize he is just talking to keep them distracted, it's too late and Gauls surround and kill them.
The Narrator describes situation; after Ambiorix's massacre of the Romans, he garners a great deal of support from other Gauls, many of them believing that they are only a step of way from driving the Roman Republic's influence out of Gaul. The gathered tribes marched hurriedly west towards Cicero, another Roman winter camp that has no idea about the fate of their comrades. We are then shown a green forest with a few dozen Roman soldiers cutting down wood for fortifications for the camp. The Narrator will then say that the siege of Cicero will start off just as the siege of Sabinus and Cotta's camp had begun.
Among those cutting the wood are the main characters Titus Pullo and Lucius Vorenus and their friends and fellow centurions Gaius Antistius Reginus and Titus Sextius. As the camera focuses on these men they begin discussing the recent Roman failure in Britain. Soon the discussion turns into each man telling his accomplishments, Pullo and Vorenus doing the most. They begin to argue, but yells interrupt them. A large group of Gauls have charged the Romans cutting wood. While most of the Romans are armed with axes, the Gauls have the element of surprise and superior numbers. After the four characters kill a few Gauls, they among others retreat to the safety of the winter camp.
The Romans quickly take to arms and mount the ramparts and with great difficulty manage to hold out for the day, the Gauls placing all their hopes on speed. During the night, using the collected timber, the Romans worked with incredible speed and built 120 towers and beefed up other defenses. The legate in command of Cicero, Marcus Silanus, offers large rewards to any man who will sneak a letter to Julius Caesar. But all those sent are intercepted by the Gauls. The siege continues the next day with even more Gauls attacking. Similar attacks are made day after day. But there was no let up of work in the nights, whatever is needed to meet the sieges during the day were made in the night. Even the sick and wounded are not allowed to sleep, as every man is needed.
The Gauls sends an envoy into Cicero to parley with Marcus Silanus. They claim the whole of Gaul is in arms, Germans have crossed the Rhine into Gaul, and all the other winter camps are under attack. Ambiorix steps forward to tell of Sabinus and Cotta's deaths. The Gauls claim they will let Silanus and his men leave in peace as long as they stay out of Gaul. Silanus doesn't fall for the same trick Sabinus did and the fighting resumes. Using the knowledge Roman prisoners, the Gauls began constructing towers larger than Cicero's ramparts and made grappling hooks and protective sheds.
The third day of the siege a man from Ambiorix's band of Gauls, Vertico, deserted to the Roman's side. Using one of Vertico's slaves, Silanus managed to get a letter to Caesar who immediately set out to Cicero. Caesar sent a Gallic cavalryman loyal to him to Cicero with a letter of encouragement to Silanus. The cavalryman was afraid to get too close, so he tied the letter to a javelin and threw it over the walls into the camp where it was discovered quickly.
On the seventh day of the siege a strong wind blew and the Gauls started to sling red-hot missiles and incendiary darts at the huts in the camp. They quickly caught fire. With cries of joy as if they'd already won the Gauls brought up a great number of towers, sheds, and ladders to the walls of the camp. Even as fires burned around them and they knew their baggage and possessions were going up in smoke, the Roman troops held their positions.
When the fighting is the fiercest near the fortifications Titus Pullo charges out at the enemy. Not to be out done, Vorenus follows. In pursuit are a few other Roman centurions; among them are Reginus and Sextius. After a valiant fight that results in Vorenus saving Pullo's life in battle, and Pullo saving Vorenus' life afterwards and both men killing several Gauls. It is at this time that Caesar and his legions arrive, the Gauls learn of his arrival and lift the siege, charging into Caesar's armies. While Caesar had significantly less troops, he was winning. The Gauls finally broke ranks and fled when the Romans in Cicero left the camp and charged the Gauls. A great number of the Gauls were killed, but those that got away, among them Ambiorix, Caesar decided not to pursue until the dead were buried.
Caesar will immediately speak with the legate Marcus Silanus to praise him and his men for holding out against such superior numbers. Caesar speaks to centurions whose outstanding valor had been mentioned to him by Silanus. Among them are of course Pullo and Vorenus, Caesar telling them that two of his senior grade centurions died in the battle and after hearing of their deeds, wants them in his legion, the 10th. They of course accept the offer of a lifetime. Caesar sends for more forces, sending a letter to Gnaeus Pompey to send recently formed legions stationed in northern Italy, and he commands nearby legions lead by Gaius Fabius, Marcus Crassus, and Labienus to join him. It is two days before the dead are buried and the fresh legions arrive. Leaving two legions at commanded by Marcus Crassus, Caesar’s forces head east towards the territory of the Eburones. Among them is Silanus’ legion so they can get revenge, Labienus’ legion, and Gaius Fabius’ legion.
They enter the territory of the Nervii, one of the tribes that were part of the coalition of Gauls led by Ambiorix. They send envoys begging for mercy, which Caesar grants after he makes them give up two hundred hostages. Meanwhile, in Eburones territory, Ambiorix’s second command, a man he forced to step down as chief, Catuvolcus, is furious at Ambiorix. It turns out Catuvolcus was against attacking the Romans entirely. He and Ambiorix argue, trading insults, but it is pointless with arguing Ambiorix and he orders Catuvolcus to put the camp on full alert. Catuvolcus makes a comment about Ambiorix hiding in his shed after he leaves.
At night Catuvolcus rides off to meet with Julius Caesar. Catuvolcus explains how he used to be chief until Ambiorix usurped his rule. He says he fully supports Rome. Caesar makes a proposal that if Catuvolcus leaves an opening in the Eburones defenses that would allow for Romans to win with as least casualties as possible, he will only kill Ambiorix and most of his supporters, thus leaving Catuvolcus in charge with the support of Rome. Catuvolcus of course agrees and gives the information to Caesar, and after he’s gone Caesar has Cassius send word to tribes all over that if they attack the Eburones on a certain day, the Romans will not stand in their way. The following day several Gallic tribes attack the Eburones. By dusk they leave, much of what the Eburones had gone or destroyed. Caesar arrives and before discussing anything with Catuvolcus, asks where Ambiorix is. Catuvolcus tells him he has fled, heading across the Rhine. Catuvolcus is then killed and the legions wipe out whatever is left of the Eburones.
So they do not lose Ambiorix, Titus Pullo and Lucius Vorenus lead a large team of cavalry across the Rhine. They catch up with a small part of Ambiorix’s forces, all cavalry. They defeat them just to learn Ambiorix had fled moments ago. They finally catch up with him and engage in a battle. While Ambiorix has two hundred more cavalry than the Romans, the Romans are far superior and it is quickly apparent the Gauls will lose the battle. Ambiorix slips out, but Pullo and Vorenus spot him and pursue. Ambiorix leads his horse to a cliff, the steed stopping and sending him flying over. Ambiorix manages to get a hold of the reigns and pulls himself to safety. While Ambiorix escaped being dropped onto jagged rocks, Pullo and Vorenus have caught up with him and are waiting for him. Ambiorix desperately tries to fight back, but the centurions behead him, the last remnants of the Gallic rebellion are no more.