In the 5th century AD, as the Roman Empire begins to pull back from Britannia, the native Woads, led by Merlin, launch a revolt against their declining power. Amidst this turmoil, a band of Sarmatian knights, who had been incorporated into the Roman army for their formidable cavalry skills, find themselves under the command of Artorius Castus, known as "Arthur." After serving their time for Rome, they are eager to return home. However, Bishop Germanus (Ivano Marescotti) summons them for one last mission after they heroically save his convoy from a Woad attack.
The Bishop orders them to rescue a prominent Roman family threatened by the aggressive Saxons led by the merciless Cerdic (Stellan Skarsgård) and his son Cynric (Til Schweiger). Among the family members is Alecto (Lorenzo De Angelis), a potential candidate for the papacy, making his capture a priority for the Saxons seeking ransom. Apprehension grows among the Sarmatians as they feel abandoned by the Romans, realizing that the Saxons pose a grave threat to all south of Hadrian's Wall, erected three centuries prior to defend against northern threats.
Despite his men's dissatisfaction, Arthur assembles a formidable group, including Lancelot (Ioan Gruffudd), Tristan (Mads Mikkelsen), Galahad (Hugh Dancy), Bors (Ray Winstone), Gawain (Joel Edgerton), and Dagonet (Ray Stevenson), to undertake the perilous task. Lancelot expresses his discontent with Arthur's decision to accept a Christian mission, arguing that conflict is an eternal cycle without end.
As they journey onward, Arthur's troop is ambushed by the Woads but narrowly escapes thanks to a timely call from Merlin, who does not seek Arthur's death. Upon their arrival, they encounter Marius (Ken Stott), the Roman patriarch, who is opposed to leaving his estate, having enslaved the local populace. Appalled by this injustice, Arthur liberates Guinevere (Keira Knightley) and her brother Lucan (Johnny Brennan) from captivity. He confronts Marius, demanding that he either leave with the knights or face imprisonment, ultimately taking control of the homestead.
Pursued relentlessly by the Saxon forces, Arthur’s party seeks refuge in the vast forests. Tension arises as Lancelot grapples with his burgeoning feelings for Guinevere, while Arthur's own attraction to her grows. Merely seeking a divine connection with their roots, Merlin suggests that Arthur forge a coalition army with the Woads to combat the Saxons.
Chaos unfolds as Marius attempts an insurrection but is swiftly dealt with by Guinevere. With his death, Marius's soldiers shift allegiance to Arthur, who learns unsettling truths about Roman betrayal, particularly regarding his mentor, Pelagius (Owen Teale), leading to a deeper estrangement from the Roman way of life.
The bonds of loyalty are tested as Arthur leads his men against the Saxon siege. In a valiant stand at Badon Hill, aided by innovative tactics from Merlin, they gain the upper hand. Amidst fierce battles, personal sacrifices are made, as Dagonet bravely disrupts the Saxon ranks, and in the chaotic fight, Lancelot saves Guinevere but suffers a mortal wound.
Ultimately, Arthur confronts Cerdic, emerging victorious, and thus receiving the honor of king of Britain from Merlin after marrying Guinevere. With Romans withdrawing and the Saxons thwarted, Arthur pledges to unify the Britons against future threats, as the spirits of fallen knights are immortalized in tales shared across generations, signified by three untamed horses galloping across the land in symbolic remembrance.
