In 400 A.D., a decadent and weakened Roman Empire nevertheless remains the greatest power on earth. But to the east a fierce new people has arisen, the Huns. An ancient prophecy tells of a great king who will unite the Huns and challenge Rome for control of the world.Young Attila (Rollo Weeks) grows up hearing his father King Mundzuk (Jolyon Baker) talk about future Hun conquest of "the western lands". After their tribe is massacred by another, 10-year-old Attila and his older brother Bleda are raised by their paternal uncle King Rua (Steven Berkoff). The soothsayer Galen (Pauline Lynch) tells Attila of her vision that he is the great king of the ancient prophecy.Years later the adult Attila (Gerard Butler) rides alongside Rua, and a trembling Rome still stands only because the Huns don't know how weak the Empire has become. To deal with the threat, Empress Dowager Placidia (Alice Krige) is forced to turn to former army general Flavius Aetius (Powers Boothe). He had once lived among the Huns, and knows Rua. Placidia had imprisoned Aetius for trying to depose her while she was regent for her son, the current incompetent Emperor Valentinian III (Reg Rogers). Now she releases Aetius, who is joyfully reunited with his daughter Lydia (Kate Steavenson Payne).Attila attacks a village inside Roman territory and brings home several captives, including the woman N'Kara (Simmone Jade McKinnon). Having seen no soldiers during the raid, he concludes that Rome is in fact defenseless and ripe for the taking. Rua, however, refuses to risk waking a sleeping lion. Galen encourages Attila to keep faith in her vision of the god of war. After a hard battle the god fell asleep and dropped his sword, which fell to earth. Whoever finds it will rule the world. While Bleda (Tommy Flanagan) is heir to the childless Rua's throne, Attila is in Galen's visions.Given first choice of Attila's Roman spoils, Bleda spitefully takes N'Kara. Galen urges Attila to be patient; his time will come. Under Hun tradition a woman doesn't belong to a man until she's borne him a child, so Galen also helps N'Kara avoid becoming pregnant by Bleda.The Huns continue making raids into Roman territory. Valentinian wants to strike back, but Aetius advises that shifting Rome's limited military forces to stop the Huns would invite a Visigoth attack. The Huns turned the Visigoths against the Empire in the first place, causing the split into the West ruled by Valentinian and the East ruled by Theodosius (Tim Curry). Rome must avoid fighting the Huns and the Visigoths at the same time.Aetius stages and then "foils" an assassination attempt on Valentinian, making it look like Placidia's doing to regain her former power as regent. The emperor agrees not to act against his mother right away. Aetius then visits Rua and tells him the Visigoths plan to attack both the Huns and Rome. Rua tells Aetius about Attila and Bleda's rivalry, and points out N'Kara among the people.Led by Aetius and Attila, the Romans and Huns soundly defeat the Visigoths in battle. After surrendering to his longtime enemy, Visigoth king Theodoric (Liam Cunningham) asks if "the girl is well". Aetius refuses to say anything about Lydia. Attila accompanies Aetius back to Rome, where Valentinian's sister Honoria (Kirsty Mitchell) loses no time successfully seducing the Hun. After several weeks in Rome, Attila receives the news that Rua is dead and returns home.Bleda is about to be crowned king when Attila challenges him to a duel. That night Galen admits to Attila that Bleda forced her to give him an undetectable poison for the contest. The next morning as the brothers shoot arrows at each other from horseback, Galen performs a magic ritual to transfer her strength to Attila while she absorbs the effects of his injuries. Attila thus survives being shot twice by poisoned arrows, and finally kills Bleda. Galen dies, and Attila claims the throne and N'Kara.Attila unites the various Hun tribes under his rule. N'Kara dies in childbirth. At Galen's grave Attila finds the fabled sword of the god of war. He proceeds to conquer other nations, including parts of the Eastern Roman Empire. Rome remains free, but if the Eastern capital of Constantinople falls then Rome will be the only target left. Aetius prepares to visit Constantinople and deal with the situation personally.Honoria is caught plotting against Valentinian, who sends her with Aetius to the East. Aetius tells Lydia that Honoria is being exiled. Attila's lieutenant Orestes (Andrew Pleavin) tells his king that the Huns are wondering why Attila doesn't attack Rome. Some believe it's because he's afraid of Aetius.In Constantinople a surprised Honoria is given to a convent. To break Attila's grip on the Eastern Empire, Theodosius has devised a plan to assassinate him. One of Theodosius' slave girls, Ildico (also Simmone Jade McKinnon), closely resembles the late N'Kara. Aetius offers to help her avenge the Huns' destruction of her village, and they slip into Attila's camp disguised as commoners. After Theodosius' assassin fails in his mission, Aetius' presence is discovered. Aetius declines Attila's offer to join him, and Attila declines Aetius' request for a peace treaty.Attila sees Ildico among his slaves. A romance blooms, and they are about to marry when a message arrives from Honoria. She offers to marry Attila if he'll rescue her from the convent, with half the Western Roman Empire as her dowry. Attila now has a perfect pretext to attack Rome. Theodosius gleefully sends Honoria home, and wishes Aetius the best of luck against the Huns.While Attila marches on Rome by way of Gaul, Aetius prepares to make a stand at the Roman-held Gallic city of Orleans. Placidia advises him to have a contingency plan in case the city falls. Aetius dismisses the idea of an alliance with the Visigoths, but Placidia knows that an unsuspecting Lydia is Theodoric's daughter. Attila captures Orleans, and Aetius asks Lydia to travel with him to Gaul.Valentinian is relaxing in a bath when Placidia brings in Honoria to beg her brother's forgiveness. After sending his sister back to prison, he confronts his mother about the assassination attempt. Mother and son then realize how Aetius has manipulated them.The Roman and Visigoth armies meet in Gaul. As expected, Theodoric wants his daughter back in exchange for allying with Rome. A last-ditch plea from Aetius fails, and he returns Lydia to the father he stole her from years ago.The Huns engage the Romans and Visigoths near the village of Chalons. When the Huns are about to win, a distraught Aetius declares he'll die there on the battlefield, and as his last action orders his lieutenant to kill Theodoric. The death of their king galvanizes the Visigoths into wresting victory from the jaws of defeat. Theodoric's son Thorismund (Mark Letheren), the new Visigoth king, then ends the alliance and heads home to defend his crown against his scheming brothers. But a stunned Attila, whose holy sword was shattered in the battle, decides to withdraw. Aetius returns to a hero's welcome in Rome.Attila marries Ildico, who takes her revenge by poisoning his drink on their wedding night. With Attila dead, Valentinian decides that Aetius is no longer needed and kills him.Without Attila, there was no one capable of uniting the nations outside Rome's borders. Without Aetius, there was no one capable of protecting Rome. Within a generation the Western Roman Empire fell, and the era later known as the Dark Ages began.