Shortly before the coronation of Romulus Augustulus (Thomas Brodie-Sangster) as Emperor in AD 475, Ambrosinus (Ben Kingsley), a Druid belonging to a clandestine brotherhood, is on an adventurous quest to find Julius Caesar's sword. This secretive mission also includes finding a ruler who deserves the power that comes with the sword. Acting as the tutor to young Romulus, he often appears as a magician, though his illusions are typically mere tricks. Romulus's father, Orestes (Iain Glen), governs Rome but lacks the title of Emperor itself.
The day prior to Romulus's big day, Odoacer (Peter Mullan), the commander of the barbarian Goths allied with the Roman forces, demands one-third of Italy from Orestes, who firmly refuses. That same day, Romulus crosses paths with Aurelius (Colin Firth), the leader of the Nova Invicta Legion, and an old friend of Nestor (John Hannah), a senator.
On the evening following Romulus's coronation, as Aurelius is appointed head of the royal guard, Rome falls under attack during the Battle of Ravenna. Many of Aurelius's men perish, leaving him shocked and presumed dead. Tragically, Orestes and his wife meet their demise at the hands of Odoacer's lieutenant, Wulfila (Kevin McKidd), who captures Romulus. Now under the rule of Odoacer, who has seized control of the Western Roman Empire, a plot is laid out to execute the young ruler. Convincing Odoacer that killing Romulus would only make a martyr of him, Ambrosinus manages to spare the boy's life. Romulus is banished to Capri along with Ambrosinus, guarded by Wulfila and his warriors, imprisoned in a villa built by Emperor Tiberius centuries before.
With the guidance of Ambrosinus, Romulus stumbles upon a hidden chamber in the villa, where a statue of Caesar holding his sword, forged by a skilled smith after military conquests, stands. An inscription nearby hints that the sword was made for "he who is destined to rule," which Romulus takes as a sign of destiny.
Aurelius and three surviving legionaries—Vatrenus (Owen Teale), Batiatus (Nonso Anozie), and Demetrius (Rupert Friend)—along with Mira (Aishwarya Rai), an agent from the Eastern Roman Empire, rescue Ambrosinus and Romulus from captivity. They originally intended to help Romulus escape to a seaport that promised safe passage to Constantinople. However, they soon realize that the Senate (including Nestor) and the Eastern Empire have betrayed them, siding with Odoacer. The Ambassador from the Eastern Empire informs them that the promise of safe passage for Romulus is no longer valid, and when Mira learns of her master's disloyalty, she chooses to join Romulus and Aurelius instead.
As Odoacer’s forces attack, Aurelius and his companions fight bravely. The tide turns as Aurelius slays Nestor, while Mira takes down the ambassador. The survivors manage to escape. Ambrosinus encourages Romulus to seek refuge in Britain, where the Legio IX Hispana might still remain loyal, being away from the turmoil in Rome. However, the team is pursued by Wulfila and the Goths, who are now eager to seize Caesar's sword upon hearing of the prophecy.
Navigating through the Alps and across the English Channel, the group reaches Hadrian's Wall but initially finds no sign of the legion until a farmer identifies himself as Kustennin (Robert Pugh), the commanding general of the force. Faced with the collapse of Roman authority in Britain, the legion had chosen to settle as farmers, having formed families and ties to the land, reluctant to confront the warlord Vortgyn (Harry Van Gorkum), who aspires to dominate all of Britannia.
During their time in a nearby Celtic village, Romulus befriends a young girl named Igraine (Alexandra Thomas-Davies), who remains blissfully unaware of his royal identity. Vortgyn, previously an adversary of Ambrosinus, covets Caesar's sword for his ambitions to rule all of Britain and aligns with the Goths. A conspiracy forms between Wulfila and Vortgyn to exchange the sword for the captives Romulus and Ambrosinus.
As Vortgyn intimidates a young Igraine into betraying Romulus, tragedy strikes as he kills the blacksmith’s family in a cruel ultimatum. When Igraine informs the villagers of the grim fate awaiting Romulus, Aurelius reveals Romulus’s true identity as the Emperor. Angered, the blacksmith seeks vengeance, leading Aurelius and his compatriots to rally an army for a decisive confrontation against Vortgyn.
Before departing the village, Igraine gifts Romulus with a suit of Roman armor that belonged to her late brother. At Hadrian's Wall, as Aurelius wields Caesar's sword, he bravely leads the charge against Vortgyn's forces. For a moment, the battle seems lost until the remnants of the Ninth Legion reappear, claiming their battlefield identity and rallying to support their ancient cause. As two sides come to a temporary standstill, Ambrosinus strikes down Vortgyn in a fierce display of resolve, incinerating the enemy as the tension eases.
Following a fierce clash where Aurelius battles valiantly against Wulfila, Romulus confronts him, fatally plunging Caesar's sword into his enemy’s chest. Witnessing the destruction around him, Romulus disowns the sword he had sought for so long, embedding it in the stone as a symbol of relinquished power.
Years later, Ambrosinus, now known as Merlin, guides a young boy to the battlefield, recounting the legendary events. Remarkably ageless since the war, Merlin shares that Aurelius married Mira and they raised Romulus as their own, who blossomed into a wise ruler. Taking Igraine as his bride, he adopted the name "Pendragon." The boy, Arthur, recognizes Romulus as his father and Igraine as his mother, concluding with a striking image of Caesar's sword lodged firmly in stone, the original inscription faded but now merely reading "E S CALIBVR."
