Centuries ago, the Amazons, a proud and fierce race of warrior women, led by their Queen, Hippolyta, battled Ares, the god of war, and his army. During the battle, Hippolyta specifically targeted and beheaded her son Thrax, whom Ares forcibly conceived with her and who is fighting for his father. Hippolyta then defeated Ares, but Zeus stopped her from delivering the death strike. Instead, Hera bound his powers with magic bracers so that he was deprived of his ability to draw power from the psychic aura of violence and death he could instigate, effectively rendering him mortal, and only another god could release him. In compensation, the Amazons were granted the island of Themyscira, where they would remain eternally youthful and isolated from Man in the course of their duty of holding Ares prisoner for all eternity. Later, Hippolyta was granted a daughter, Princess Diana, whom she shaped from the sand of the Themyscirian seashore and gave life with her own blood.

Over a millennium later, an American fighter pilot, USAF Colonel Steve Trevor, is shot down in a dogfight and crash-lands on the island, where he soon runs afoul of the Amazon population, including the warlike, aggressive Artemis. Steve and Diana meet and fight, and she defeats him, taking him to the Amazons. After interrogating him with the use of the Amazons' golden lasso, Hippolyta decides he is not an enemy of the Amazons and as such, tradition dictates that an emissary be tasked to ensure his safe return to his own country. Diana volunteers, but is assigned to guard Ares's cell instead since her mother argues that she has not enough experience in dealing with the dangers of the outside world. Diana defies her mother and, her face hidden by a helmet and her guard duty covered by her bookish but kind-hearted Amazon sister Alexa, participates in contests of strength and wins the right to take Trevor back to his home.

In the meantime, the Amazon Persephone, who has been gradually seduced by Ares, kills Alexa and releases him. With the additional task of capturing Ares, Diana brings Trevor to New York City, where he volunteers to help Diana on her quest. An investigation uncovers a pattern of violence created by Ares' presence that will lead to him given time, and the pair goes out to a bar while they wait. After some heavy drinking, Trevor makes a pass at Diana. They argue outside, but are attacked first by thugs and then the demigod Deimos. Deimos kills himself to prevent being interrogated, but Diana and Steve find a clue on his body that leads them to a concealed gateway to the underworld guarded by members of a still-extant ancient cult of Ares.

Once there, Diana attempts to subdue Ares, but he summons harpies that knock her unconscious, prompting Trevor to save her instead of stopping Ares. Meanwhile, Ares performs a sacrifice to open a gate to the Underworld where he persuades his uncle Hades (who has made Thrax his slave) to remove the bracers, though Hades does not tell Ares that the ultimate cost of removing the bracers would be Ares' own death in combat. Later, Diana regains consciousness in a hospital and is furious that Trevor saved her rather than stop Ares. Trevor argues against her abuse with his own criticism of the Amazons' self-imposed isolation and their generalizations about men, and reveals how much he cares about her.

Ares and his army attack Washington, D.C. Trevor and Diana arrive to battle Ares and are soon joined by the Amazons. While Ares manages even to summon the Amazons long dead from the Underworld to fight their own sisters, his scheme is stopped by Alexa, a member of the undead host, who reveals to Artemis a chant which nullifies Ares's control over them. The undead then turn on Ares but are destroyed by his powers. Hippolyta faces Persephone in combat and kills her, but in her dying breath, Persephone makes the queen realize that in shutting the Amazons away from the world of men, she has denied them the chance to live their lives as women.

Meanwhile, the President of the United States is influenced by Ares's power and orders a nuclear missile against Themyscira, assuming the island nation to be the source of the attack on Washington. This act of supreme aggression increases Ares's power, but Trevor takes the invisible jet and shoots down the missile just before it hits the island. Finally, after a brutal beating at Ares's hands, Diana finally outmaneuvers and beheads him in the same manner that Hippolyta beheaded Thrax. As Trevor arrives back at the scene, Diana finally accepts him, as the two share a kiss, and the crowd of Amazons begin cheering. Subsequently, Ares is condemned to the underworld to attend Hades as a slave alongside his son.

Later on Themyscira, in memory of Alexa, Artemis takes up the hobby of reading (with severe difficulty). Hippolyta realizes that Diana misses both the outside world and Trevor, and to make her happy again, she gives her daughter the task of being a channel for 'communication between men and women'. Diana accepts and returns to New York, where she enjoys the company of Trevor. Their relationship comes with the understanding of her larger duties, such as when Diana sees Cheetah robbing a museum and she excuses herself to stop the supervillainess as the newly christened Wonder Woman.