As a young child, Diana participates in an athletic competition on Themyscira against adult Amazons. After being knocked off her horse, Diana takes a shortcut and remounts but misses a checkpoint. Antiope removes her from the race for cheating, explaining anything worthwhile must be obtained honestly, while her mother Hippolyta advises her to be patient in her pursuit of glory and honor.

In 1984, Diana is working at the Smithsonian Institution in Washington, D.C., while secretly performing heroic deeds as Wonder Woman. New museum employee Barbara Minerva, a shy geologist and cryptozoologist, has trouble getting noticed by her co-workers, but quickly finds a friend in Diana.

After the FBI asks the museum to identify stolen antiquities from a robbery that Wonder Woman recently foiled, Barbara and Diana notice one of the artifacts, later identified as the Dreamstone, has a Latin inscription claiming to grant the holder one wish. Neither openly takes the inscription seriously, but Barbara wishes to become like Diana and unintentionally acquires the same superpowers, while Diana yearns for her deceased lover Steve Trevor. He is resurrected in another man's body and the two are reunited at a Smithsonian gala. Failing businessman Maxwell "Max Lord" Lorenzano steals the Dreamstone, hoping to use its power to save his nearly bankrupt oil company. His wish is to "become" the stone and gain its wish-granting powers so whenever he grants someone else's desire, he can take what he wants from the wisher, resulting in worldwide chaos, destruction, and instability.

Diana discovers that the Dreamstone was created by Dolos/Mendacius, The God of Lies, also known as the Duke of Deception. It grants a user's wish but exacts an equally strong toll unless they renounce the wish or destroy the stone. Although Diana's powers and Barbara's humanity begin to diminish, neither is willing to renounce their wish. Max visits the U.S. President at the White House, who wishes for more nuclear missiles to use against the Soviets, which brings the world to the brink of nuclear war. Max also learns of a new and secret satellite system that can broadcast to anyone in the world. Since his powers are causing his body to deteriorate, he plans to grant wishes globally to steal strength and life force from the viewers and regain his health. Diana and Steve confront him, but Barbara betrays Diana and overpowers her, escaping with Max on Marine One.

Steve convinces Diana to renounce her wish and let him go, restoring her to full strength. Donning the armor of Asteria, the greatest of all Amazon warriors, Diana flies to the satellite headquarters and again battles Barbara, who has transformed into a humanoid cheetah after wishing to become an apex predator. After a brutal fight that ends in a lake, Diana electrocutes Barbara, then pulls her out of the water. She confronts Max and uses her Lasso of Truth to communicate with the world through him, persuading everyone to renounce their wish. She then shows Max visions of his unhappy childhood and of his son, Alistair, who is frantically searching for his father amid the chaos. Realize the error of his ways, Max renounces his wish and reunites with Alistair, and Barbara returns to normal. Sometime later in winter, Diana meets the man whose body Steve possessed. Meanwhile, Asteria is revealed to be secretly living among humans.