In present-day Paris, Diana Prince receives a photographic plate from Wayne Enterprises (accompanied by a letter from Bruce Wayne) of herself and four men taken during World War I, prompting her to recall her past. The daughter of Queen Hippolyta, Diana is raised on the hidden island of Themyscira, home to the Amazons, women warriors created by the Olympian gods to protect mankind. Hippolyta explains their history to Diana, including how Ares became jealous of humanity and orchestrated its destruction. When the other gods attempted to stop him, Ares killed all but Zeus, who used the last of his power to wound Ares and force his retreat. Before dying, Zeus left the Amazons a weapon, the "god-killer", to prepare them for Ares's return. Hippolyta reluctantly agrees to let her sister, General Antiope, train Diana as a warrior.

In 1918, Diana, now a young woman, rescues US pilot Captain Steve Trevor when his plane crashes off the Themysciran coast. The island is soon invaded by German soldiers, who had been pursuing Steve. The Amazons kill the crew, but Antiope sacrifices herself to save Diana. Steve is interrogated with the Lasso of Hestia and reveals that a great war is consuming the outside world and that he is an Allied spy. He has stolen a notebook from the Germans' chief chemist, Dr. Isabel Maru, who is attempting to engineer a deadlier form of mustard gas under the orders of General Erich Ludendorff. Believing Ares to be responsible for the war, Diana arms herself with the "god-killer" sword, the lasso, and armor before leaving Themyscira with Steve to locate and stop Ares for good.

In London, they deliver Maru's notebook to the Supreme War Council, where Sir Patrick Morgan is trying to negotiate an armistice with Germany. Diana translates Maru's notes, revealing that the Germans plan to release the deadly gas at the Western Front. Although forbidden by his commander to act, Steve, with secret funding from Morgan, recruits Moroccan spy Sameer, Scottish marksman Charlie, and Native American smuggler Chief Napi to help prevent the gas from being released. The team reaches the front in Belgium. Diana goes alone through No Man's Land and captures the enemy trench, liberating the nearby village of Veld with the aid of the Allied forces. The team briefly celebrates, taking a photograph in the village, where Diana and Steve fall in love.

The team learns that a gala will be held at the nearby German High Command. Steve and Diana separately infiltrate the party; Steve intends to locate the gas and destroy it, and Diana hopes to kill Ludendorff, believing that he is Ares. Steve stops her to avoid jeopardizing his mission, but this allows Ludendorff to unleash the gas on Veld, killing its inhabitants. Blaming Steve for intervening, Diana pursues Ludendorff to a base where the gas is being loaded into a bomber aircraft bound for London. Diana fights and kills him, but is confused and disillusioned when his death does not stop the war.

Sir Patrick appears and reveals himself as Ares. He tells Diana that although he has subtly given humans ideas and inspirations, it is ultimately their decision to resort to violence, as they are inherently corrupt. When Diana attempts to kill Ares with the "god-killer" sword, he destroys it, telling Diana that as the daughter of Zeus and Hippolyta she herself is the "god-killer". He fails to persuade Diana to help him destroy mankind in order to restore paradise on Earth. While the two battle, Steve's team destroys Maru's laboratory. Steve hijacks and pilots the bomber carrying the poison to a safe altitude and detonates it, blowing up the plane and himself. Ares attempts to direct Diana's rage and grief at Steve's death by convincing her to kill Maru, but the memories of her experiences with Steve cause her to realize that humans have good within them. She spares Maru and redirects Ares's lightning into him, killing him for good. Later, the team celebrates the end of the war.

In the present day, Diana sends an email to Bruce Wayne thanking him for the photographic plate of her and Steve. She continues to fight on the world's behalf as Wonder Woman, understanding that only love can truly save the world.
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