NOTE: This spoiler was submitted by Spectre.
A text scrawl tells that in 120 A.D., the Roman Ninth Legion went into Northern Britain and never returned. Shamed at this humiliating loss, the Roman empire constructed a great wall to separate northern and southern Britain. North of the wall was considered "the end of the world."
20 YEARS LATER (140 A.D.)
We see a small squad of Roman soldiers head up the river. It is quiet but they are on their toes, fearful of any attack. They continue to travel until they arrive at a small Roman base. Among the squad is Marcus Aquilla (Channing Tatum) who is actually arrived to take command of the base. Lutorius (Dennis O' Hare), who had been acting commander, shows Marcus around. Lutorius asks Marcus if he has family connections here. Marcus hesitates before saying "no."
Later, we see that the men clearly don't respect Marcus, and it is not just because he is young. His father was a top commander in the Ninth Legion who protected "The Eagle": A bronze eagle that represented Rome's honor. When the legion disappeared, so did Marcus' father and the eagle. The men think Marcus is a bad omen.
Marcus holds a meeting and noticing food hasn't arrived for the men, orders a patrol to make sure nothing is wrong. He also notes the fort is not as fortified as it should be and has work done for it to be more defensible.
Later that night, Marcus is dreaming of his father and the last time he saw him. His father gave him a carved eagle which Marcus wears on a necklace to this day. He is suddenly startled awake and sensing an attack, he gets the men up. After seeming that he got them up for nothing, rebels attack the base. Thanks to Marcus having them prepared and his defensive measures, the Romans repel the army with few casualties.
The following morning it is clear that the men respect him now. However, there joy is short lived. The rebel Britons have returned in droves with the lost patrol as their hostages. The rebel leader admonishes the Romans for their murders of their sons and the rapes of their daughters. He then proceeds to execute one of the men. Marcus takes himself and half of the men out to rescue the men, as it was his fault they went out in the first place.
They get to men and rescue them, but have to retreat when the rebels ride towards them with bladed chariots. To buy his men time, Marcus spears the chariot carrying the rebel leader, killing the driver. The chariot flips toward him and lands on top of him.
Marcus wakes up in the villa of his Uncle Aquilla (Donald Sutherland). His wound was too great, and he had to be moved here so he would live. He is patched up though now walks with a limp.
Lutorius comes to visit Marcus and tells him several things. One, the men of his command have been honored with a new rank for their brave defense of the fort and that Marcus himself has received a military honor (think Medal of Honor or similar medals for equivalent). However, he has been honorably discharged due to his wound. Marcus is anguished at not being a solider anymore and screams loudly. His uncle tells his servant to let him be for "There is no medicine for that."
Marcus and his uncle go to some games where gladiatorial games take place. A young slave Esca (Jamie Bell) is pitted against a gladiator. Esca is not in the mood to fight, and would rather die than be a slave any longer. The fighter beats up Esca and goes in for the kill. The crowd wants it, but Marcus changes the tide so they will let him live.
At home, his uncle tells him he bought him a slave. It turns out to be Esca. When left alone, Esca tells him that he hates everything Marcus and Rome stand for, but since he saved his life, he is honor bound to serve him. He gives Marcus his father's dagger as a sign of his loyalty.
A senator and his son come to see Marcus and his uncle. During dinner, the senator speaks of rumors the Ninth Legion eagle had been spotted in Northern Britain, and though they would love to retrieve it, Rome will not gamble thousands of soldiers on it. The senator's son then speaks rudely of Marcus' father, which leaves him enraged. He tells his uncle he will find the eagle and restore the honor of their family. He will take Esca, for he knows the land and the language.
They move into Northern Britain and search for weeks and ask around about the eagle, getting nowhere. Over a campfire one night, Marcus confides in Esca about how he sometimes dreams that he will find his father out there. The reason why the eagle is so important, for it represents Rome. Everywhere it goes it meant "Rome did this." Esca counters this with a story of his own. Seven years ago his family and others rose in defiance of Rome for wanting to defend their land. Esca's brothers and father were killed. His mother too, by his father's hand so the Romans wouldn't savagely rape her before killing her. "Rome did that too." Esca says.
While traveling one day, they are attacked by some rogue warriors. Marcus and Esca kill them all, but the last one is a small boy no older than ten. Esca is shocked and lets him go only for Marcus to throw a knife and slit the boys throat. He tells Esca "Next time show no mercy." Meaning they cannot slip up for anyone or they risk death.
After another man gives them no info, Marcus threatens the man into giving the information. They travel over a mountain into the woods. There they are attacked by a man named Guern (Mark Strong) who is actually a survivor of the Ninth Legion. He was taken into a tribe as one of their own and now has a wife and two sons. He shows them where the Ninth had it's last stand. Guern explains the Seal people killed the Ninth and that Esca knows about them.
While traveling from the battleground, Marcus fights with Esca over keeping secrets. Esca gets the upper hand only to realize they are surrounded by the Seal Tribe. The Seal Prince asks who Esca is. He tells him of his slain father and that Marcus is his Roman slave. The Seal Prince invites Esca to camp.
The Seal Chief is humored at the idea of a Briton with a Roman slave and takes Esca into the tribe. Marcus is treated poorly and sees the other side of the coin. He almost loses his life when he subtly flirts with some village girls, one turning out to be the Chief's daughter and thus the Seal Prince's sister. Esca beats Marcus and offers to kill him. The Seal Prince declines saying Esca is his guest, and Marcus is his property. Marcus swears he will kill Esca after that.
Marcus watches one night as the tribe does a tribal dance with The Eagle as a main part of the ceremony. He loses composure and races down to the beach to retrieve it only to be knocked out. Later that night, Esca finds Marcus and gives him a sword. Everything he did was an act to build trust with the tribe. They race to get the eagle.
They get to it, but the Seal Chief and his men are there anticipating a betrayal. Esca kills the men and Marcus kills the Seal Chief after seeing his father's ring on the man's finger. The Chief in his last breath says his father "Was just another dead Roman. Died begging for his life."
Marcus and Esca have to leave before the Seal tribe realizes what happened. They are almost stopped by a small boy Esca befriended, but he convinces the boy to stall. They get a head start before the Seal Prince realizes what happened and sends a search party after them.
The two travel until one of the horses collapses in exhaustion, and they have to mercy kill it. They eventually have to abandon the other and travel on foot. Marcus at this point is tired and delirious and cannot go any further. He tells Esca to leave and take the eagle back to Rome. Esca refuses, saying he gave his word. Marcus then frees Esca, but Esca promises to return with help.
Marcus hallucinates about his father again until he sees something in the distance. It is Esca, with Guern and twenty or so survivors of the Ninth Legion. Guern tells Marcus his father died defending the eagle, and he was no coward. He and the other men are here to redeem themselves. As the Seal Tribe surround them, Marcus order them into defensive positions around the eagle.
The Seal Prince admonishes Esca as a traitor and kills the young boy Esca befriended by slitting his throat for his supposed betrayal. The Seal tribe charge the legionaries, and they begin to fight. Many are killed and the Seal Prince goes to kill Esca, but Marcus stops him and drowns him in the stream. At the end, all the Seal tribe men are dead. Guern had been killed, and all but about six former legionaries survived. They burn Guern in a funeral pyre with a prayer for Romans and Britons alike who fought with honor for what they believed in. On the pyre, Marcus puts his father's carved eagle, and Esca places his father's dagger, as a way to let go of the past.
Marcus and Esca return to Rome with The Eagle and present it to the incredulous Senator. The Senator says the honor of the Aquilla family has been restored. The Senate also wants to rebuild the Ninth and will offer the command to Marcus (though it is clear he no longer has a desire to serve as a soldier). The Senator's son makes a snide comment on how Marcus and a slave (Esca) brought the eagle back, but Marcus says Esca is not a slave and has more honor than he will ever have in his life. Marcus and Esca leave the room, and when Esca asks where they should go next, Marcus says "You decide."