On a stormy night, the son of the god Zeus and human Alcmene (Karolina Szymczak) was born. His mother named him Hercules, which meant "glory of Hera", to appease the goddess herself. Hera saw the boy's birth as an insult and planned to have him killed. Two serpents emerge from the eyes of a Hera statue and slither their way to the child, but the young Hercules snaps their necks with his god-like strength.
As an adult, Hercules (Dwayne Johnson) went through his famed Twelve Labors. He treads through a swamp to take on the Lernaean Hydra, slicing off their heads one by one. He faced the Erymanthian Boar, taking it down with his club. His most well-known labor was the slaying of the Nemean Lion, a beast whose hide was so powerful that no mere weapon could penetrate it.
The person telling this story, Iolaus (Reece Ritchie), who is also Hercules' nephew, is tied up and hanging over a wooden pike right below his groin. He is taken prisoner by pirates that have invaded Macedonia. The pirate leader starts to burn the rope that holds Iolaus, not believing a word of his story. The leader fails to see Hercules standing behind him, wearing the skin of the Nemean Lion on his head and boasting that he killed the beast with his bare hands...or so they say. Hercules charges forward with his comrades - the seer Amphiaraus (Ian McShane), Autolycus (Rufus Sewell), Atalanta (Ingrid Bolso Berdal), and Tydeus (Aksel Hennie) - and takes out all the pirates and pushes Iolaus to safety before he falls onto the pike. Hercules finishes off the leader with one powerful swing of his club.
Hercules and his allies go to a tavern to eat and drink. They are approached by a woman named Ergenia (Rebecca Ferguson), asking Hercules to help her father protect the kingdom of Thrace from a warlord. If he succeeds, he and his friends will be rewarded their weight in gold. Autolycus pushes a plate of food to Hercules and tells him to eat up.
The crew travels to Thrace. Ergenia's son Arius (Isaac Andrews) runs up to Hercules, eager to meet this legendary warrior after hearing about his Twelve Labors. General Sitacles (Peter Mullan) escorts Hercules and his crew to meet Ergenia's father. Hercules is asked if he's ever been this close to royalty. He recalls three years earlier when he traveled to the kingdom of King Eurystheus (Joseph Fiennes), bringing him the heads of the Hydra. He is also reunited with his wife Megara (Irina Shayk) and his three children. Hercules then experiences a vision of his family screaming, and the walls covered in blood.
Hercules is brought before Lord Cotys (John Hurt), Ergenia's father. He requests Hercules' help in fighting off an army led by the dreaded warlord Rhesus. He takes a look at Atalanta and says this mission holds no place for a woman, though she proves him wrong immediately by shooting two arrows at the helmets of two soldiers, throwing it out of their hands. It is said that his soldiers are centaurs that laid waste to Cotys's forces. Hercules agrees to train Cotys's soldiers in combat. It starts with the men holding their shields up for protection, but they are easily knocked down when Tydeus charges at them.
At night, Arius wanders around when he hears a snarling noise. He looks in the darkness to see Tydeus, who lunges at him, but the boy runs away. He is found by Hercules, who tells Arius that he and his friends found Tydeus as a child in Thebes after a war overtook his home, and to this day, Tydeus does not speak a word. Hercules brings Arius back to his mother, who is in the hospice tending to the wounded and sickly. As a gift, Hercules gives Arius the tooth of the Nemean Lion, which excites the boy.
Hercules leads the soldiers to the Bessi Heartland. Iolaus tries to join the soldiers, but Hercules refuses to let him fight. The area has already been ravaged, with countless bodies sprawled everywhere, including young children, and the heads of men mounted on spikes for display. The soldiers come across several "fresh" bodies, which turn out to be savages that set up a trap for them. The soldiers line up with their shields up, and Hercules breaks off the head of one of Atalanta's arrows. One of the savages charges toward Hercules, and he punches the savage so hard that he flies backwards, with the arrowhead lodged in his skull. The Thracians take on the savages, while Hercules and his crew fight back, leaving Iolaus and Cotys to be guarded. Atalanta wields her bladed bow while firing arrows without even having to look behind her. Tydeus charges like mad at the savages while Autolycus uses his weapons against them also. When it looks like the savages are overpowering them, Hercules and Amphiaraus ride a horse-drawn chariot with blades, killing every last savage in sight. The soldiers mourn their fallen, and Iolaus covers Hercules' wounds so that the others won't see him bleed like a mortal. Cotys calls out Rhesus, knowing he is behind this, unaware that the warlord is watching from afar.
Ergenia tends to Hercules' wounds back in Thrace. When she asks him if he has had a family before, he dodges the subject and sends her away. That night, he hears screaming. He wanders out and sees dead bodies everywhere, including those of his wife and children. Hercules is then faced with the three-headed dog of Hades, Cerberus. It is only a vision, however, as Tydeus comes by to snap Hercules out of it. This, as Amphiaraus states, has happened before.
The next morning, Ergenia finds Hercules' friends and asks Iolaus to tell her the truth about what happened to Hercules' family. Iolaus begins to say that it happened with Hera driving him mad, but Ergenia dismisses that as myth. Regardless, Autolycus tells Ergenia that they all have reason to trust Hercules.
Hercules leads the soldiers to Mount Asticus, where they see what appear to be centaurs in the distance. They run toward the Thracians, but they turn out to be simply men riding horses. Rhesus (Tobias Santelmann) makes his presence known to the soldiers, though Hercules is not the least bit intimidated by him. The Thracians take on Rhesus's men, proving more powerful than before. The opposing army fires flaming arrows at the Thracians. Amphiaraus, thinking his time is coming, stands in there, welcoming death, but not a single arrow hits him. Rhesus charges toward Hercules on his horse, but Hercules grabs the horse and flips him and Rhesus over before sneering, "Fucking centaurs."
Rhesus and his men are taken back to Thrace where they are mocked and have garbage thrown at them. A feast is held, with Rhesus being chained up as a prisoner. Ergenia feels pity for him, and she tries to bring him water, but Cotys won't let her. Hercules gets Ergenia alone after seeing her look at Rhesus, and it is learned that it was Cotys that orchestrated the whole war to gain power and control over the kingdom, with Rhesus merely having gotten himself involved. Ergenia only wants Arius to be safe, since he will ascend to the throne after Cotys dies. She asks Hercules to take the boy with him, but he says no. He goes to confront Cotys, who shows no remorse over his actions and offers Hercules a place in his army. Hercules, of course, refuses.
The crew is given their gold, but Hercules wants to stay and help Thrace. Autolycus chooses to leave, taking his share of the gold with him, despite the others pleading with him to stay. Atalanta, Amphiaraus, and Tydeus agree to help Hercules fight Cotys. However, when they all go to face him, they are captured, along with Ergenia and Arius.
Hercules is chained up in the dungeon while his friends are locked in cells. He sees what appears to be Cerberus, but they are merely three vicious wolves. In enters King Eurystheus, who turns out to be in league with Cotys. It is also revealed that he drugged Hercules the night his family was murdered, sending the wolves to attack and kill them, because Eurystheus saw Hercules as a threat to his kingdom when his people cheered his name louder than Eurystheus. To make things worse, Cotys orders Ergenia to be executed right there in the dungeon. Amphiaraus tells Hercules to remember all that he has accomplished, and to remember who he is. He cries, "Who are you?!" And the man bellows, "I AMMMMM HERCULEEEES!" He breaks off the chains and swings at the executioner, stopping him from taking off Ergenia's head. She runs to free the others while the wolves attack Hercules. Hercules slams one against the wall, then breaks the jaw of another, and then finally kills the last one with the Nemean Lion's tooth (Arius passed it back to him while being captured).
With everyone, including Rhesus, freed, the Thracian soldiers try to attack. One hurls a spear that passes through fire at Amphiaraus, which he foresaw as his death. He stands there, ready to accept his fate, when Hercules grabs the spear and throws it back at the soldier. Hercules then goes after Eurystheus, who tries to weasel his way out of it by saying Cotys corrupted him. He begs for forgiveness, but Hercules grabs Eurystheus's dagger and impales him with it. Sitacles then appears and grabs Hercules with his whip, but Iolaus impales him from behind, now ready to join the fight.
The final battle comes outside the palace, where Cotys stands before his soldiers. They bring out Arius, threatening to kill him if Hercules doesn't surrender. A soldier holds a knife at the boy's neck, but the man is killed by a knife, thrown by Autolycus, who has decided to return for his friends. The Thracians fight Hercules and his crew, while Arius runs back to his mother. Tydeus is struck by an arrow, but he doesn't let that stop him as he slays every soldier in his path. Hercules retrieves him, where Tydeus merely utters Hercules' name before dying. The crew spills oil and fire to block the Thracians, and Hercules begins to push the statue of Hera from its base. Cotys begins shouting at Hercules, calling him a coward and saying his family deserved to die. Hercules finally manages to break the statue from its base, causing it to come crashing down, crushing some soldiers while the head rolls and smashes into Cotys, throwing him over the edge of a cliff.
The surviving soldiers lay down their weapons and bow before Hercules, chanting his name. The rest of his crew stands behind him. Amphiaraus delivers the closing lines by saying that he prefers this tale of Hercules to the myth. He says that you don't need to be a demigod to be a hero, but you just have to believe you can be a hero. He adds, "But what do I know? I'm supposed to be dead."