Shortly before the coronation of Romulus Augustulus (Thomas Brodie-Sangster) as Emperor in AD 475, Ambrosinus (Ben Kingsley), a Druid and part of a secret brotherhood protecting the hidden sword of Julius Caesar (he is on a quest to find the sword itself and a ruler fit enough to wield it), has become the tutor to the young Romulus Augustulus. He at times gives the impression he is a magician, but his "magic" is usually revealed to be simple trickery. Romulus' father Orestes (Iain Glen) rules Rome but is not Emperor himself. On the day before Romulus' coronation, Odoacer (Peter Mullan), commander of the barbarian Goths allied with Rome, demands a third of Italy from Orestes, but is rebuffed. The same day, Romulus meets the general of the Nova Invicta Legion, Aurelius (Colin Firth). Nestor (John Hannah) is a Senator and an old friend of Aurelius.The night after the coronation (also when Aurelius is appointed the head of Romulus's guards), Rome is attacked by the Goths in the Battle of Ravenna. Most of Aurelius' men are killed, and Aurelius is stunned and left for dead. Orestes and his wife are killed by Odoacer's lieutenant, Wulfila (Kevin McKidd), who captures Romulus. The next day, Odoacer, now the ruler of the Western Roman Empire, plans to have Romulus killed. Ambrosinus convinces Odoacer to spare him (Killing Romulus will make a martyr out of him) and Romulus is exiled to Capri along with Ambrosinus, guarded by Wulfila and his men. His prison is a villa constructed more than four centuries earlier by the emperor Tiberius.With Ambrosinus' help, Romulus discovers a hidden chamber within the villa. He comes across a statue of Caesar holding his sword, forged by a Chalybian smith after his military campaigns in Britain. Writing near the statue's feet proclaims the sword was made for "he who is destined to rule". This is interpreted as a prophecy and Romulus keeps the weapon.Ambosinus and Romulus are rescued from Capri by Aurelius and three surviving legionaries, Vatrenus (Owen Teale), Batiatus (Nonso Anozie) and Demetrius (Rupert Friend), accompanied by Mira (Aishwarya Rai), a female Indian agent of the Eastern Roman Empire. They had planned to rescue Romulus from Capri & They take Romulus to a seaport where the Eastern Roman Empire has promised safe passage to Constantinople. They learn that the Senate (including Nestor) and the Eastern Empire have betrayed them and sided with Odoacer. The Ambassador from the Eastern Empire says that Romulus no longer has safe passage to Constantinople and withdraws the services of Mira. when Mira finds out that her master has abandoned Romulus and Aurelius, she chooses to go with them, instead of going back to her master's empire. Odoacre's forces attack Aurelius and his men, whom they fight off. Aurelius kills Nestor, Mira kills the ambassador, and the survivors escape.Ambrosinus persuades Romulus to seek refuge in Britain, where the Legio IX Hispana (Ninth Legion) may remain loyal, being far from the events. They are followed by Wulfila and the Goths, who covet Caesar's sword after learning of the prophecy. Crossing the Alps and the English Channel, the party travels to Hadrian's Wall and initially find no evidence of the legion until a farmer reveals he was its commanding general Kustennin (Robert Pugh). With the collapse of Roman support of Britain, the legion had decided to disband and settle as farmers. Most of the men in the legion had married and had families and did not want to antagonize the region's powerful warlord, Vortgyn. Vortgyn rules the Southern kingdom and wants control over the whole of Britannia (including the Northern parts beyond Hadrian's wall).. During their stay in the small Celtic village, Romulus meets and befriends a young girl named Igraine (Alexandra Thomas-Davies), who is unaware of Romulus' true identity.Vortgyn (Harry Van Gorkum), revealed to be an old enemy of Ambrosinus (Vortgyn wanted the sword, but Ambrosinus was looking for a righteous ruler worthy of the sword), desires Caesar's sword as he aspires to rule the whole of Britain and he allies with the Goths. Wulfila makes a deal with Vortgyn to trade the sword for Romulus and Ambrosinus. Vortgyn confronts Igraine outside the village and makes her tell the village to surrender Romulus, killing the village blacksmith's family in a show of force. When Igraine tells the villagers of what has happened, Aurelius confesses that Romulus is the Roman emperor. The blacksmith demands revenge for his family, and Aurelius and his men decide to lead an army to Hadrian's Wall to face Vortgyn's armies in a final battle.Before leaving the village, Igraine gives Romulus a suit of Roman armor which belonged to her brother, which he has since outgrown. At Hadrian's Wall, Aurelius, wielding Caesar's sword, leads his men and a group of Celtic warriors against Vortgyn's forces. Vortgyn's forces begin to overwhelm Romulus' forces until the rest of the Ninth Legion, having taken up their old Roman arms and uniforms, arrive to help. The two warring sides cease their hostilities upon seeing Ambrosinus holding Vortgyn's golden mask, after burning him alive at a sanctuary of his secret brotherhood. After having fought and severely injured Aurelius, Wulfila is confronted by Romulus, who stabs Wulfila in the chest with Caesar's sword, killing him. Repulsed by the deaths in the battle, Romulus disposes of his sword, lodging it in a large stone.Many years later, Ambrosinus, now known by his Druid name, Merlin, takes a young boy to the battlefield to describe the legendary events. Merlin, who has visibly aged little since the battle, says that Aurelius married Mira and the two raised Romulus as their own son. He became a wise ruler, took Igraine as his wife and adopted the name "Pendragon". The boy, Arthur, recognizes Romulus as his father and Igraine as his mother. In a final scene, Caesar's sword is shown embedded in the stone, covering the original Latin inscription, now only reading "E S CALIBVR".