Apparently historians agree that the classical 15th century take on King Arthur was based on a real hero named Artorius Castus who lived a 1,000 years earlier during the Roman occupation. At the beginning of the film, Lancelot takes up this story in a voiceover, saying by 300AD the Roman Empire stretched from Arabia to Britain but they were not satisfied and had attacked the powerful Sarmatians to the east. At the end of the 4th day of intense fighting, only a few Sarmatian cavalry were left. However the Romans had been impressed with their bravery, and had spared their lives on two conditions first they were incorporated into the Roman military and second their children and childrens' children were also pressed into service.
So in 452AD we see Lancelot as a young lad being taken into service - around the same time a young Arthur was being groomed to take over from his father as leader of the Sarmation Knights. With his father away a lot, the priest Pelagius had became his beloved guardian, prior to travelling to Rome to spread his message of free will and freedom which was blasphemous to the Pope.
The Knights under Arthur serve their 15 years in Roman Britain south of Hadrians Wall and the film jumps to 467AD as Arthur and the remnants of his Knights ride out to meet Bishop Germanius and his Roman escort who is coming to present their freedom papers for travel throughout the Roman Empire. The Bishops escort is attacked by Merlin's Woads and the Knights arrive in time to turn the tables on the attackers. They return to the Hadrian's Wall garrison, but not before Arthur surprisingly spares the life of a Woad -watched from the trees by Merlin.
Arthur offers Germanius his quarters to refresh himself. Whilst there he notices a disc of Arthur's depicting Pelagius and contemptuously tosses it on to the floor, breaking it. He is further horrified when he comes to the meal to find a round table where he could not assume a position of head of the table. This had been introduced by Arthur who felt to follow Pelagius teaching, it was necessary for all to have equal placements at the table. However there are many empty places and all that survive are Arthur (Clive Owen), Lancelot(Ioan Gruffudd),Tristan, Gwain, Galahad, Bors (Ray Winstone) and Dagonet.
Bishop Germanius firstly dismays the knights by reporting that the Romans would now be pulling out of Britain and that a large Saxon army had landed and would probably take over the country. So for what had they risked their lives? He then dismisses the others to explain to Arthur alone the doubled edged reason for his visit : the Marius Honorius family, in particular his son Alecto who was the Pope's favourite godson and pupil and a possible papal successor, were in danger of capture by the Saxons, so Arthur and his men have been ordered by Rome to effect a rescue before their freedom papers can be served. Arthur is furious on behalf of his fellow Knights at this betrayal of their freedom, being sent on their most dangerous mission yet as they would need to travel north of Hadrian's wall. The wily Germanius says it's for the Church whom Arthur loves and surely they would not balk at rescuing a young boy.
Arthur goes into the courtyard as Bors' partner Vanora sings a hypnotic song about home. Arthur is spotted but he dashes their hopes when he explains their mission. Dagonet implicitly trusts Arthur and prepares for the mission. The others in varying degrees of reluctance and anger follow his example and the next day they set off through Hadrian's Wall.
Meanwhile Cerdic's Saxons have landed. They burn every building and kill everyone they come across. They are joined by a Woad spy who tells them of the important Roman family. Cerdic (Stellan Starsgard) sends his son Cynric with his men to capture the family and then rejoin his father at Hadrian's wall.
Arthur's men go deep into Woad territory to reach the Roman family and are soon ambushed by the Woads. As they move in for the kill, Merlin gets word of the Saxon invasion, and to their disgust, he pulls the Woads back from the ambush and Arthur and his men are bemused to find they are free to continue their journey.
They reach the family and the father Marius at first refuses to go but Arthur forces them for his men's freedom sake. He then learns of Marius' harsh dealings of the peasants which cuts across his Pelagian ideals and he sets free their local leader and orders them to prepare for the march south. They are about to depart,hearing the Saxons' drums, when Arthur becomes suspicious of a bricked up building. There he finds Woads with all but two starved and tortured to death. He rescues the two survivors - Guinevere (Keira Knightley) and a young boy - and takes them on their escape route from the Saxons. Marius is furious but Arthur fells him and they all leave, with Marius threatening reprisals when they get back to Hadrian's wall.
Guinevere recovers on the journey and plays with both Arthur's and Lancelot's feelings, telling them that the Britains not the Romans are their real heritage.
Arthur knows they must rest and they spend the night in a forest. He follows Guinevere who brings him to Merlin. He accuses Guinevere of leading him into a trap but she says Merlin is not there to kill him. Merlin explains that his men see Arthur more Britain than Roman and as their only leader against the Saxon invasion.
As day breaks Marius grabs the Woad boy and threatens to kill him. Guinevere calmly shoots Marius with her bow and arrow and his men are given the choice of surrendering or dying. They choose the former. Tristan arrives and says the Saxons are much closer and they must leave. As they journey, Arthur seeks Alecto's forgiveness for his father's death. However Alecto calmly says not only had his father lost his way but stuns Arthur by saying he fights for a Rome that doesn't exist and that Pelagius had been excommunicated and killed under Germanius' orders for heresy the previous year.
They reach a glacier and the Saxons are right on their tail. They dismount from their wagons and Arthur leads them all carefully over the cracking ice. He then sends the rest on while the 7 Knights and Guinevere take up a position at the end of the glacier with bows and arrows to await the Saxons.
As the Saxons approach, the Knights starting killing those on the flanks forcing the advancing Saxons to bunch but still the ice doesn't break up. Arthur realises the ice is not going to break and gives the order to draw swords, Dagonet rushes forward with his axe and starts hacking away at the ice. The archers try to protect him but eventually Saxon arrows get through and he dies just as his last blow breaks up the ice. Many Saxons drown and the rest cannot follow them as a sad Arthur and his men retrieve Dagonet's body, catch up with column and lead them to safety. Cynric and his survivors return to his father who strips him of his leadership and humiliates him.
A very belligerent band of Knights snatch their freedom papers from Germanius and then bury Dagonet. After the funeral Guinevere sits with Arthur and presses the point of how his allegiance should really be with this island especially now that the Romans have tricked him.
That night Guinevere comes to his bed and while they are making love, Arthur is summoned to the wall where he witnesses the Saxons' arrival.
Arthur is now convinced that he should remain but he releases the Knights from their allegiance to him and sends them away with Germanius and the Roman garrison while he and the villagers prepare their defences. They are joined by Merlin and the Woad archers led by Guinevere.
Cerdic is desperate to meet this Arthur, the name he has continually heard since he landed, and under cover of a truce he meets him face to face, finally deciding that Arthur is the first man on this island worth killing.
As Arthur goes back behind Hadrian's wall, an invigorated Cerdic watches as the gates to the Wall mysteriously open. He sends an advance party of Saxons (Cyrnic's survivors but without Cyrnic) forward to soften up Arthur's men. Meanwhile as the retreating Romans march away, the remaining Knights' horses are unsettled by the sound of the Saxons beating on their war drums. The Knights look at each other and know they have to re-join Arthur but this time it is of their own volition.
The first wave of Saxons are annihilated by a combination of the Woad archers and the Knights so Cerdic leads his remaining men into the final bloody battle. This time the big siege engines are brought in by Merlin to support Arthur. Lancelot and Cyrnic kill each other, Cerdic pointedly kills Tristan in the sight of Arthur who charges Cerdic and they fight to the death - Cerdic's death. The Saxons are annihilated at the place now known as Badons Hill.
Arthur is grief stricken over Lancelot's death and says he's let them all down but the survivors Gwain, Galahad and Bors all stand by him. In the final scene Merlin marries Arthur and Guinevere and Arthur is proclaimed King to the joy and support of Woads and Knights alike.