300 begins with a voice-over that details the arduous journey of Spartan boys transforming into seasoned Spartan warriors. As a child is born, a Spartan baby inspector scrutinizes the newborn atop a cliff, where the remnants of those who were deemed unfit lie below. Any sign of weakness or illness results in a lethal consequence for the infant. The boys are trained in combat from a young age, and at the age of seven, they are separated from their mothers to embark on rigorous training and initiation rituals before they are allowed to return home.
As life in Sparta unfolds, a Persian messenger appears, seeking an audience with King Leonidas, portrayed by Gerard Butler. The messenger aggressively demands that the people of Sparta bow to Xerxes, the Persian god-king. Queen Gorgo, played by Lena Headey, interjects, and the messenger is dismissive of her, leading to a tense confrontation. Defiantly, King Leonidas shoves the messenger and his guards into a black pit, declaring, "This is Sparta!"
Sparta's law dictates that the King must consult the Oracle and the Ephors, who are a grotesque assembly more myth than man, before waging war. The Oracle, trapped in a life of beauty and despair, is sought by Leonidas as he attempts to gather support for a campaign against Xerxes' impending army. The Ephors refuse to allow war due to an upcoming celebration, frustrating Leonidas who feels helpless in the face of such encroaching danger.
Amid political intrigue, councilman Theron, who is secretly aligned with the Persians, influences the Ephors against the King. With his loyal Captain at his side, Leonidas assembles a force of just 300 men to confront the invading Persian forces, declaring their path towards the north where a strategic choke point awaits them.
As the Spartan warriors set out, they are joined by the Arcadians, bolstering their numbers in the face of overwhelming odds. The Queen seeks to rally a more substantial army to aid her husband, but is thwarted in her efforts by the councilmen's indifference. Meanwhile, as the Spartan contingent solidifies their defenses, they face a harrowing storm that damages the Persian fleet but does little to diminish the enemy's advantage.
With no surrender in sight, the narrative intensifies as Xerxes sends a new envoy demanding compliance from the Spartans. Demonstrating their fierce independence, one Spartan sends the envoy back with a brutal message. When Ephialtes—a disfigured man desperate to join the Spartans—approaches the King with critical knowledge of a hidden pass for the Persians, Leonidas must turn him away due to his inability to fight alongside the others, igniting deep-seated anger within Ephialtes.
In battle, the 300 Spartans initially hold their ground, showcasing their superior combat skills by decimating the first wave of Persian forces without loss. However, their victory is shadowed by the overwhelming numbers that follow, with an ominous barrage of arrows darkening the sun itself.
The mighty Xerxes, described as a towering, god-like figure, attempts once more to persuade Leonidas to kneel. The proud King, however, confronts him defiantly, yet understanding the cost of their resistance. Each Spartan falls in their valorous fight, but not before Leonidas gives an emotional farewell to one of his men, urging him to carry the tale of their extraordinary stand against tyranny.
In a parallel narrative, Queen Gorgo handles betrayal and deceit among the councilmen. After a chaotic turn of events, she confronts Theron, culminating in a violent clash that exposes his treachery. Meanwhile, Ephialtes betrays Sparta to Xerxes, giving the Persians the edge they need.
As the Spartan warriors face encirclement, the contest reaches an agonizing point. Amid the chaos, Leonidas makes the ultimate sacrifice, fighting until his final breath. He eloquently remarks the honor he feels in standing with his brothers, leaving a powerful legacy. As the story winds down, Dilios, an injured Spartan, returns to relay the tale of bravery and sacrifice which eventually leads to renewed hope for Sparta.
In a climactic turn of events, one year later, he joins a vastly larger army, ready to re-engage and avenge the fallen against their oppressors. The spirit of Sparta ignites once again, as thousands rally to march back into battle.
