In the opening scene in modern day Paris, France, Diana Prince (Gal Gadot) visits the Louvre as a van from Wayne Enterprises pulls up. She receives a briefcase sent by Bruce Wayne. Inside is an old photograph of Diana alongside four men during World War I. With the picture is a note from Bruce saying he would like to hear the story behind the picture.
As a child, Diana (played by Lilly Aspell) grew up on the island of Themyscira alongside the mighty Amazons, an all-female warrior clan. She watches the warriors training on the field under General Antiope (Robin Wright). Diana wants to be a fighter like the rest of the Amazons, but her mother, Queen Hippolyta (Connie Nielsen) forbids her daughter from training.
Hippolyta takes Diana and tells her the story about the gods. Zeus ruled over the heavens and looked upon mankind with high regards, but Ares, the god of war, sought to corrupt mankind and lead them to tear each other apart in battle. The other gods fought back against Ares, but he killed all of them. With his dying breaths, Zeus cast Ares down into the world of man and then forged a weapon that can destroy Ares once and for all. He also gave the Amazons their home on Themyscira to hide them from the rest of the world. Hippolyta shows Diana the powerful sword known as the God Killer. With it, Ares can be killed, but Hippolyta hopes there will not come a time for Diana or anyone else to wield it.
Antiope convinces Hippolyta to let her train Diana, wanting her to be ready to face Ares, because they all know he's still out there. Hippolyta reluctantly agrees. Through the years, Antiope subjects Diana to harsh and intense training in the field. As she reaches adulthood, Diana continues training to be as fierce as the other Amazons. During one session, Antiope pushes Diana hard with her swordplay, leading Diana to defend herself by deflecting the sword with her gauntlets. This causes Antiope to fly back and hurt herself. Diana shows great remorse for hurting Antiope. She leaves in shame, but Antiope is far from upset.
Diana runs over to a cliff. In the distance, a plane passes through the barrier into Themyscira before crashing into the ocean. Diana dives into the water and swims toward the plane. A pilot named Steve Trevor (Chris Pine) is sinking as he cannot break himself free from his seat. Diana pulls him out of the plane and takes him to shore. As she becomes surprised to see a man for the first time, German soldiers following Steve make it past the barrier as well. The Amazons arrive as they find Diana with Steve, but also see the Germans. The Amazons ready their arrows, and the Germans aim their rifles. The arrows are launched and shots are fired. The Germans make it to the shore and battle the Amazons. Several Amazons are shot dead while the Germans get taken out easily. Antiope fires three arrows at once to kill three soldiers. One soldier aims his gun at Diana, and Antiope leaps in to take the shot. Diana rushes to Antiope's side. As she dies, she tells Diana it is her time to wield the God Killer.
Steve is interrogated before all the Amazons with the Lasso of Truth. He is forced to confess that he is a spy with the Allied forces. Steve explains to the Amazons about World War I and how he has learned of a sinister plan by Germany's General Erich Ludendorff (Danny Huston) and his chief chemist Dr. Isabel Maru, AKA Dr. Poison (Elena Anaya). They are concocting a new bio-weapon, which Steve claims is unlike anything he's seen before and tells the Amazons they are in more danger than they realize. Steve acted as a spy in the German army before stealing Maru's notebook and flying away with it as he destroyed the Germans' other planes until they followed him to the island. Diana realizes this must be the work of Ares.
Steve is allowed to bathe in the Amazons' fountain to replenish himself. Diana walks in and accidentally sees him nude. She asks what "that" is, and Steve says it is his father's watch, which he says he follows to let him know what to do in the day. Diana is confused by the device and its uses.
Still fearing that Ares is behind the ongoing war, Diana grabs her shield, lasso, and then takes the God Killer sword from its holding, along with her battle suit. She goes to tell Steve she will help him get home if he takes her to Ares. The two head for the shore but are approached by Hippolyta and other Amazons. Diana is insistent that she goes out there to stop Ares and end the war. Hippolya tearfully embraces her daughter one last time and tells her she has always been her greatest love, but today she is her greatest sorrow. She gives Diana Antiope's tiara as a parting gift, but warns her that if she leaves she may not be able to return to Themyscira. As Diana and Steve depart, Hippolyta confides to an Amazon that Diana can't know what she really is, or else Ares will find her faster.
Diana and Steve arrive in London. The two meet with Steve's secretary, Etta Candy (Lucy Davis). Before they can deliver Maru's notebook to Steve's superiors, they find Diana an appropriate outfit to wear so she can blend in for the time. Diana and Steve are then cornered by some men in an alley. They draw their guns, but Diana beats them all down, even letting Steve punch the last guy out.
Ludendorff meets with Maru in her lab. She has developed a new type of gas for the general so that he may maintain his super strength. Maru then comes across a formula on her table, which she thinks is the breakthrough they've been looking for.
Diana and Steve head on over to a meeting held by Sir Patrick Morgan (David Thewlis), who is planning to negotiate an armistice with Germany. The men in the room are immediately distracted by Diana's presence. Steve is forced to take her outside. Later, he presents Sir Patrick with the notebook. Since the writing is in Sumerian, Steve has Diana (who speaks hundreds of languages) translate it for the other men. They learn that Ludendorff and Maru are planning to create a new kind of hydrogen-based mustard gas that would be far more lethal to the soldiers. After hearing that the British general is planning to send his troops to the Western Front and go in to most likely get killed, Diana becomes enraged and tells the general that a good leader would fight alongside his men. Steve pulls Diana out again as she tells him that she believes Ludendorff is Ares in disguise. Steve confides to Diana that he will personally take her to the Front.
Steve brings Diana to meet his teammates. They first stop at a pub and find Sameer (Said Taghmaoui), a spy who is instantly attracted to Diana. Next is Charlie (Ewen Bremner), whom Steve says is a skilled marksman, but they find him getting beaten up by another man for accidentally taking his glass. The man attempts to shoot Charlie, but Diana intervenes and knocks the man down. The four then head out to meet The Chief (Eugene Brave Rock), a smuggler.
Ludendorff meets with some of his comrades to discuss what he has planned. After they express some doubts and lack of confidence in his work, Ludendorff and Maru demonstrate their work by throwing the gas bomb in the room, killing all the men.
The team makes it to the Front. They walk through the trenches, and Diana speaks to a local woman who is distraught over what has been taken from them by the German army. Not wanting to wait any longer, Diana dons her tiara and steps out into the battlefield with her armor. The German soldiers begin to fire upon her, but she easily deflects their bullets with her gauntlets. Diana proceeds to beat down the enemy soldiers while Steve and his team follow. Once she takes out most of the soldiers on the ground, the men help Diana leap to a sniper tower, which she instantly demolishes. The townspeople celebrate and cheer for Diana.
Steve later learns that Ludendorff will be attending a gala the next day. He has Etta locate the area for him, but Sir Patrick urges Steve not to go ahead and do it. Steve naturally ignores him. He then tells Diana about the gala and how she would be unprepared for it, so he shows her how to dance. They go up to a room to be alone, and they share a kiss.
The team heads over to the gala in disguise. Steve pretends to be a German officer with Sameer being his driver as they improvise entering without an invitation. Diana finds a snooty woman and steals her dress. Steve attempts to sweet-talk Maru to get more information, but he becomes distracted when he sees Diana enter in the blue dress. Diana sees Ludendorff and gets ready to kill him, but Steve stops her from messing up the plan. However, Ludendorff proceeds to launch gas bombs into the nearby town. Diana runs ahead but is too late. She sees the townspeople dead in the streets and can only look on with despair. Steve runs by and Diana is angry with him for not letting her kill Ludendorff before this happened.
It is learned that Ludendorff plans to use a plane to drop a number of gas bombs down on London. Diana rushes to an airbase where the German soldiers are gearing up for their attack. Diana beats down some soldiers before making her way to Ludendorff. He inhales Maru's strength gas and fights Diana. Ludendorff puts up a good fight until he runs to the roof. Diana bursts through and impales Ludendorff with the God Killer. He dies but Diana sees that the soldiers are still pushing the gas bombs into the plane.
Steve shows up and Diana is confused as to why the war isn't over if she killed Ares. Steve says that maybe the war isn't a result of a god but rather that there are just bad people out there. He leaves, and Diana looks back inside Ludendorff's office to see Sir Patrick. Diana realizes it is him that is really Ares. He tells Diana how he only provided secret influence to people like Ludendorff and Maru to get their work going, but it was their own evil, as well as the darkness of other men, that led to the war. Diana tries to kill Ares with the sword, only for him to destroy it. He tells Diana that it is she who is the real weapon forged by Zeus. She is the real God Killer. Using the Lasso, Ares shows Diana what can happen if she joins him and if they rid the world of mankind. She refuses, and they begin to fight.
Steve and his team get past the Germans. They consider shooting the plane down, but the explosion would kill anyone within 50 Miles. As Diana and Ares' fight extends onto the tarmac, Steve runs by Diana, says something quick, and he gives her his father's watch. He hijacks the bomber plane and flies it away. As he gets high enough, Steve aims his gun at the bombs. He takes a moment to consider what he's doing, but he knows what he has to do. He shoots the bombs, sacrificing himself as the plane blows up. Diana sees this and yells in agony. Ares only uses this to try and sway her even more to join him as he dons his armor. Ares brings Maru before Diana and removes Maru's mask to reveal her disfigured face. He tries to convince Diana to kill her. Diana lifts up a tank, but she recalls what Steve just told her. He said he can save the day, but she can save the world. He told her he loved her before taking the plane. Diana drops the tank and lets Maru get away. Diana tells Ares that she believes in love. Ares fires a blast of energy at Diana, but she absorbs it with her gauntlets and directs it back toward Ares, destroying him for good. The soldiers are then broken free of their corruption.
Everyone celebrates the end of the war. Diana sees a wall of the fallen heroes, including Steve.
Back in the present, Diana sends a message to Bruce Wayne, thanking him for the photograph and letting her keep a piece of Steve. Her voice-over then reassures that she will continue dedicating her life to justice. With that, Diana leaps across the sky to continue her work as...WONDER WOMAN.