West Africa, 1823 – An opening crawl states that the kingdom of Dahomey has crowned a young king, Ghezo (John Boyega). Their enemies, the Oyo Empire, have teamed up with the Mahi people to capture Dahomey villagers and give them to European slave traders. The Oyo have an advantage with their guns and horses. The Dahomey have a group of female amazon warriors, the Agojie, led by General Nanisca (Viola Davis), who are set to protect their kingdom.
The Agojie infiltrate a Mahi village, slaughtering the soldiers before coming across the captured villagers and freeing them. In the morning, the Agojie mourn their dead and escort the captives back home. The warriors return home before more Oyo arrive, where Ghezo considers Nanisca to be his most trusted soldier.
A young woman, Nawi (Thuso Mbedo), is brought to a man who would marry her, even though he is older and brutish. When Nawi doesn’t respond to the man, he slaps her, and she responds by pushing him hard enough for him to fall over. He is angered and refuses to marry her. Nawi’s father brings her to the Agojie and offers her to the king. She is taken in by an Agojie soldier, Izogie (Lashana Lynch).
Oyo soldiers, led by General Oba Ade (Jimmy Odukoya), come across the corpses of their comrades. Oba Ade insults them for letting their fellow soldiers get murdered by women, and they plan their next move against Dahomey.
Ghezo addresses his council to discuss the Oyo threat. Nanisca also brings up the slave trade and says it casts a dark shadow on their kingdom to be selling off their own people. She proposes that they deal in gold or palm oil for trading. Ghezo tells Nanisca to show him how much palm oil they can produce before he makes his decision.
Nanisca’s right-hand woman Amenza (Sheila Atim) addresses the new Agojie recruits, telling them they will fight and potentially die for their kingdom. Several women leave, but a large group, including Nawi, choose to stay. Izogie then tells Nawi to go bathe, so she does and speaks personally to Nanisca.
The women begin their training. Nawi does not understand why they begin with ropes, feeling she should start with a bladed weapon until Nanisca allows her to try on a wood statue to decapitate it. The sword is too heavy for Nawi, and Nanisca chops the head off with one swing. At night, Izogie speaks to Nawi and talks about how her mother allowed men to abuse her until she fought back. The women continue training later on by running, firing rifles, and learning combat skills.
Nawi plays a prank where she sets off a small explosive on one of the statues. She takes the blame for it as Nanisca chastises her for it. Later, Izogie tells Nawi about how Nanisca’s deeds are considered legendary since she rose up to her ranks after being a captive left for dead.
Amenza visits Nanisca in her hut, then tells her, using special nuts, that she sees somebody from Nanisca’s past coming back for her. Outside, Nanisca watches as Nawi practices her sword-wielding skills on one of the statues.
The Oyo arrive at the kingdom’s gates for a confrontation with Ghezo. When Oba Ade steps forward, Nanisca becomes horrified, as Oba Ade raped her repeatedly when she was a captive. They order tributes and demand 40 Agojie warriors, but Ghezo only allows 20. Nanisca tells Amenza that her vision was true.
Portuguese settlers arrive in Africa, led by Captain Santo Ferreira (Hero Fiennes-Tiffin) and his friend Malik (Jordan Bolger), the latter whose mother was Dahomey. When Malik sees the captured natives, he feels horrified. The men join the scene as the Agojie confront the Oyo. Nanisca says they will not give them tributes and instead delivers Oba Ade the severed heads of his men. A fight breaks out between the two tribes, wherein Nawi sees an opportunity to free the captives, which Nanisca says deviates from their plan. After they escape, Nanisca chews Nawi out for it, but Amenza privately tells Nanisca that it was SHE who deviated from the plan, as she was more concerned with killing Oba Ade.
Ferreira and Malik meet with Oba Ade and his soldiers as they are discussing plans for the Oyo to team up with other tribes to invade Dahomey and capture more villagers to sell off. Malik appears uncomfortable with the news. He later goes by the water to bathe, where he is found (in the nude) by Nawi. She accuses him of being a slaver, but he denies it and tells her about his Dahomey heritage. He gives her a piece of jewelry before she retreats back to the others.
The kingdom holds a small tournament where the Agojie trainees go through an obstacle course involving thornbushes, climbing a wall, fighting enemies, and finally decapitating one of the statues. Nawi comes out as the victor, and Ghezo acknowledges her skill and talent. Later, while tending to Nawi’s wounds, Nanisca feels something on the girl’s arm. She goes back to her room and is followed by Amenza, as Nanisca asks her about a child that she had given up long ago, because she is beginning to suspect that Nawi might be that child.
Ferreira speaks to Ghezo about his plans to potentially cease Dahomey’s part in the slave trade, telling him that his riches come from the slaves, and without them, his kingdom would be nothing and unprotected. Ghezo retorts that if the Europeans are no longer of use to him, then they are also unprotected.
Izogie senses that Nawi might like Malik after watching them speak discreetly while Ferreira was talking to Ghezo. They later meet in secret, where Nawi gives Malik a totem to keep him safe. When he says that he has nothing to give her, she simply takes one of his knives.
Nawi goes to bathe and encounters Nanisca, who chastises her for sneaking off for Malik. After arguing, Nanisca approaches Nawi and tells her about the baby she gave up. She was a product of rape, and Amenza helped Nanisca hide her and give her away. Nanisca remembers cutting the child’s left arm and sticking a shark tooth in it. Nanisca cuts into Nawi’s arm and pulls out the tooth, confirming that Nawi is indeed her daughter, and therefore the daughter of Oba Ade as well. Nawi leaves in disbelief.
The following morning, Nanisca and some of her soldiers keep an eye out on the Oyo camp. They set fire to their gunpowder and cause multiple explosions, scaring their horses away and leaving them vulnerable. The Agojie launch an attack, leading to a fierce and bloody battle that leaves many Oyo and Agojie dead, and leads to some of the women, including Izogie and Nawi, to get captured. On a wagon heading away, Nawi finds out woman’s binds have come loose, and she encourages her to roll off the cart and run back to Dahomey. While captive, Nawi helps Izogie try to fix her broken arm.
The young woman returns to Dahomey and informs Nanisca about the Oyo’s plan to retaliate against the Agojie. Ghezo promotes Nanisca but forbids her from acting any further, but she and Amenza go behind his back to rescue the others.
When the European slavers come for the captives, Izogie and Nawi initiate a fight against them, but Izogie is fatally shot and dies in Nawi’s arms as she wails for her sister. Nawi is imprisoned, but Malik goes to visit her and tells her how the captives are bound for England, and he wishes to protect her. They are implied to make love after.
Nanisca leads the warriors into the European’s camp for an attack. They set fire to their camp and attack the men before freeing the captives. Malik turns on Ferreira and frees the captives, causing them to drag him into the water, where they beat him and hold his head underwater until he drowns. Nawi helps Nanisca engage in battle as she finally goes to face off against Oba Ade. The man attempts to beat Nanisca, but she overpowers him and impales him, finally killing him. Everyone returns home, and Nawi parts ways with Malik.
The Agojie return home with their captives, and Nanisca is confronted by Ghezo. While he acknowledges that she disobeyed his orders, he is thankful to her for ending the Oyo Empire and officially promotes her to the title of Woman King. Everyone cheers for her.
Nawi goes to pay tribute to Izogie, and she is joined by Nanisca. She then addresses Nawi as her daughter. The two then go join the others to dance in celebration.