In northern Britannia, 62 AD, a tribe of Celtic horsemen is brutally wiped out by Romans led by Corvus. The only survivor, a boy named Milo, whose parents Corvus personally killed, is captured by slave traders.
Seventeen years later, in Londinium in 79 A.D., slave owner Graecus watches a class of gladiators battle, unimpressed until he sees the grown Milo, a talented gladiator the crowds call "the Celt". Milo is soon brought to Pompeii with his fellow slaves. On the road, they see a horse fall while drawing a carriage carrying Cassia, returning after a year in Rome, and her servant Ariadne. Milo kills the horse to end its suffering, and Cassia is drawn to him. Cassia is the daughter of the city governor Severus and his wife Aurelia. Severus is hoping to have the new Emperor Titus invest in plans to rebuild Pompeii, despite Cassia's warning of Rome becoming more corrupt. Felix, a servant, takes Cassia’s horse Vires for a ride only to be swallowed up when a quake from Mount Vesuvius opens up the ground under him.
In Pompeii, Milo develops a rivalry with Atticus, a champion gladiator who, by Roman law, will be given his freedom after he earns one more victory. The gladiators are shown off at a party where Corvus, now a Senator, tells Severus the Emperor will not invest in his plans but he himself will. It is revealed Cassia left Rome to escape Corvus’s advances. When an earthquake causes some horses to become anxious, Milo helps calm one down. He then takes Cassia on a ride and tells her they cannot be together. Returning to the villa, Corvus is ready to kill Milo (not recognizing him from the village massacre), but Cassia pleads for Milo's life. Milo is lashed for his actions, and Atticus admits respect for his rival as they prepare to face each other at the upcoming festival.
In the Amphitheatre of Pompeii, to punish Milo, Corvus orders him killed in the first battle, and wicked trainer Bellator convinces Graecus to sacrifice Atticus, as well. The two men, and other gladiators, are chained to rocks as other gladiators come out as Roman soldiers, to recreate Corvus’ victory over the Celts. Working together, Milo and Atticus survive the battle; Atticus realizes the Romans will never honor his freedom. During the battle, Corvus forces Cassia to agree to marry him by threatening to have her family killed for supposed treason against the Emperor. When Milo and Atticus win, Cassia defies Corvus by holding a “thumbs-up” for them to live, and Corvus has her taken to the villa to be locked up. Claiming an earthquake is a sign from Vulcan, Corvus has his officer Proculus fight Milo one-on-one. Their battle is interrupted when Mount Vesuvius erupts, creating massive tremors that cause the arena to collapse, sending Milo and Proculus crashing to the dungeons. Milo opens up the gates to allow his fellow gladiators a chance to attack; Proculus escapes, while the gladiators kill Bellator. Seeing Corvus fallen under a collapsed beam, Severus tries to kill him, but Corvus stabs him instead and escapes.
The eruption sends flaming debris raining down upon the city as the populace tries to flee to the harbor. One fireball destroys and sinks a ship, killing the escaping Graecus. Before dying, Aurelia tells Milo that Cassia is at the villa. Milo races to the villa and manages to save Cassia, but Ariadne is killed when the villa collapses into the Mediterranean Sea. Atticus tries to reach the harbor, but a tsunami created by the volcano smashes into the city, destroying the harbour and the outer walls, and smashing several ships. Reuniting with Atticus, Milo suggests searching the arena for horses to escape. As the gladiators face Roman soldiers at the arena, Cassia is abducted by Corvus after finding her parents' bodies. Atticus has Milo chase after the chariot carrying the two while he fights Proculus. Atticus is mortally wounded in the duel, but nonetheless manages to kill Proculus.
Milo chases Corvus across the city; both barely avoid fireballs, and collapsing infrastructure. Cassia manages to free herself before the chariot crashes into the Temple of Apollo. Milo and Corvus duel as a fireball destroys the temple. Cassia chains Corvus to a building, as Milo declares who he is, that Corvus killed his family and now his gods are coming to punish the Senator. Milo and Cassia ride off as a pyroclastic surge races into the city, incinerating Corvus. At the arena, Atticus proudly proclaims that he dies a free man before being consumed by the pyroclastic flow. At the city outskirts, the horse throws off Milo and Cassia. Milo tells Cassia to leave him, realising the horse isn't fast enough to carry them both. Instead, she sends the horse off, not wanting to spend her last moments running, and knowing they cannot outrun the surge. Milo kisses Cassia as the surge engulfs them. The last shot is of the duo's petrified bodies, locked in an eternal embrace.
Kit Harington as Milo, a gladiator
Dylan Schombing as young Milo
Emily Browning as Cassia, daughter to Severus and Aurelia
Kiefer Sutherland as Senator Quintus Attius Corvus
Carrie-Anne Moss as Aurelia, wife to Severus
Jared Harris as Severus, the city governor of Pompeii
Adewale Akinnuoye-Agbaje as Atticus, a gladiator
Jessica Lucas as Ariadne, a servant to Cassia
Joe Pingue as Graecus, a slave owner
Currie Graham as Bellator, a slave trainer
Sasha Roiz as Marcus Proculus, Corvus's Officer
Dalmar Abuzeid as Felix, a slave to Severus
Jean-Francois Lachapelle as Milo's Father
Rebecca Eady as Milo's Mother
Jean Frenette as Boss Slaver
Maxime Savaria as Biggest Thracian
Ron Kennell as The Weasel
Tom Bishop Jr. as Cassia's Carriage Driver
Emmanuel Kabongo as African Gladiator
Brock Johnson as Flashback Centurion
Kristina Nicoll as Rich Wife #1
Janine Theriault as Rich Wife #2
Mark Whelan as Ship's Captain
Anais Frenette as Harbour Child
Donna Christo as Harbour Mother
Thomas Stumpo as Harbour Boy
inspired by and based on the eruption of Mount Vesuvius in 79 A.D. that destroyed Pompeii, a city of the Roman Empire.