Shortly following the events of Batman v Superman: Dawn of Justice, Diana Prince, also known as Princess Diana and "Wonder Woman", receives a shipment from Bruce Wayne. The shipment contains an old photograph of Diana, wearing her Amazonian armor and standing with four soldiers during World War I.
Long ago, Diana lived on the island of Themyscira as the daughter of Hippolyta, queen of the Amazons. Diana had been interested in training as a warrior even as a child, but Hippolyta tried to dissuade her from taking up the blade. To try and curb Diana's interest, Hippolyta taught her about the origins of Man as the creations of the gods of old, and how the jealous God of War, Ares, rebelled against his father Zeus by tainting the hearts of Man and forcing them to fight and kill each other. The other gods died trying to defeat Ares, but Zeus manages to cast down the God of War. With the last of his strength, Zeus gave the Amazons the island of Themyscira and a special weapon known as a "Godkiller" to defeat Ares should he threaten the world again.
All this serves to do is ignite Diana's passion to train. She began to train in secret under the Amazon soldiers' captain, Antiope. When Hippolyta discovers their secret training, unable to dissuade her daughter, she orders Antiope to train Diana harder than any other soldier. During one particularly strenuous sparring match with Antiope, Diana accidentally unleashes a strange power that releases a powerful shockwave, leaving Hippolyta concerned that Diana would learn a dark secret...
Afterwards, Diana watches out over the sea when she sees something peculiar; a strange flying machine crashing into the seas surrounding Themyscira. When she goes to investigate, she finds a man in the wreckage and rescues him. Unfortunately, more men converge on Themyscira, chasing the man Diana had just rescued. The Amazons engage the invaders, who use strange weapons that make sharp sounds and can kill the Amazons at a distance. Diana and the man she rescued fend off the invaders with the other Amazons, but Antiope dies saving Diana from an attack from one of the invaders.
Cross with the man for having brought conflict to Themyscira, Hippolyta uses the Lasso of Hestia to interrogate him. He reveals his name as Steve Trevor, a spy for the United States of America working with Britain. Beyond Themyscira, a war is waging between Britain and Germany; a war to end all wars. The Germans were enlisting the aid of a scientist named Isabel Maru, also known as "Dr. Poison", to create deadly chemical weapons for General Erich Ludendorff. Steve had taken a notebook containing Maru's research and fled from a German weapons facility when he was chased into Themyisciran territory while trying to return the notebook to the British in the hopes of stopping the development of Maru's chemical weapons.
Ultimately, the Amazons decide to let Steve go so his disappearance would not attract any further attention. Diana, however, senses something is deeply amiss: the description of the war Steve gave leads her to believe it to be the work of Ares. Determined to slay Hades and stop the war, Diana takes the lasso, the Amazon's sacred armor, and the "Godkiller" sword to save the world. Hippolyta, realizing nothing could stop her daughter's course, gives her a golden tiara as a keepsake and her blessings before she and Steve leave Themyscira for the world of Man, all the while worrying about what would happen if Ares finds Diana and reveals her secret...
In London, after giving Diana some clothes to fit in and a crash course in modern culture, Steve goes to the British Supreme War Council, where a politician, Sir Patrick Morgan, is discussing the possibility of a peaceful end to the war with an armistice with Germany. The notes, according to a translation provided by Diana, contain schematics for a new variation of mustard gas using hydrogen instead of sulfur, making current countermeasures against chemical warfare useless. Steve believes the manufacture of this new weapon must be stopped, but the council is unwilling to take any action that could jeopardize the upcoming peace talks, leaving Diana cross. Nonetheless, Steve vows to stop the development of this new gas, even if he has to do so against orders.
At a bar, Steve recruits the aid of some old friends: Sameer, a Middle Eastern swindler and special agent; and Charlie, a Scottish drunk and sharpshooter. Morgan also gives his unofficial blessings to the mission and grants Steve and his group leave to travel to the Belgian front, where they meet with "Chief", a Native American smuggler aiding the Allies on the front. As they travel to the front, Diana is appalled to see the soldiers returning with grievous injuries and missing limbs, and innocent civilians suffering. When the group reaches the trenches, Diana learns that the nearby town of Veld had been taken over by the Germans, the townsfolk enslaved and suffering.
Unable to take it any longer, Diana charges headlong through the No Man's Land between the trenches to liberate Veld. Inspired by Diana's bravery and taking advantage of the diversion she provided, Steve and his group lead the Allies across, taking the German trenches and driving the Germans out of Veld. That evening, Diana and Steve share a dance, eventually realizing their feelings for each other and making love.
Eventually, the group learns that Ludendorff and Maru would be attending a gala for the German high command. Steve decides to infiltrate solo to try and find the gas so he can destroy it. Unfortunately, Diana disrupts the plan when she also infiltrates the party to kill Ludendorff, believing him to be Ares in disguise and determined to kill him and end the war. Steve tries to stop Diana, and in so doing, prevents Diana from killing Ludendorff before he can unleash his new chemical weapons on Veld, killing everyone in the town, down to the last man, woman, and child.
As Diana lashes out at Steve for stopping her and being possibly tainted by Ares, the rest of the group manage to track Ludendorff to a munitions plant. Diana and the others give chase, with Diana fighting her way through the German army, confronting Ludendorff, and finally killing him.
To her confusion, however, the Germans proceed with arming an airship with their new chemical bombs to prepare for an attack on London. As it turns out, Ludendorff was not Ares. Rather, Ares had taken on the disguise of Sir Patrick Morgan, who confronts Diana with the fact that he never coaxed Man to wage war: all he did was provide Man with ideas for new and deadlier weapons to use against each other. The decision to use those weapons was Man's alone, for Man, according to Ares, is inherently violent and corrupt. Diana tries to run Ares through with the "Godkiller" sword, only for the blade to be effortlessly crushed. Ares reveals that the true "Godkiller" was Diana, herself! After all, only a god can kill a god; thus, Zeus left the last of his children with the Amazons to use as a weapon against Ares, and that child was none other than Diana!
As Diana battles Ares, Steve's team works to stop the deployment of the chemical bombs. Steve's team destroys the weapons plant while Steve, himself, hijacks the bomber carrying the chemical bombs. Realizing there's only one way to prevent London from being bombed, Steve flies the bomber to a safe altitude before firing on one of the bombs, igniting the hydrogen-based gas and sacrificing himself to destroy the bomber and the chemical bombs.
Bereaved at Steve's death, Diana awakens her godlike powers and flies into a blind rage, laying siege to the German soldiers. Taking advantage of Diana's anger, Ares tries to coax Diana into joining him in destroying humanity to restore the Earth to a verdant paradise. To that end, he throws Dr. Maru, whom he holds as the example of humanity at their worst, to Diana's mercy. Diana, however, recalls the camaraderie she had with her team and the love she had for Steve, and ultimately decides that humanity has its redeeming values and deserve mercy. Enraged, Ares tries to kill Diana with lightning, but Diana channels it back to Ares and destroys him for good and all. With the God of War vanquished, soldiers tempted by his wicked influence who once fought on opposing sides embrace one another, and the Great War finally comes to an end.
All this transpired in 1918. It has been a long time since, and the world has changed greatly in that time. One thing remains constant, however: Princess Diana's dedication to protecting Man, tempering his violent nature through wisdom and love in the hopes of ending war forever.