The Rouran tribes are a constant threat to the ruling Chinese dynasty of the Northern Wei province in 450 A.D. so much so that the government issues a nationwide draft. Hua Hu has already fought for his country twice and has long since retired from the military but he feels obligated to serve again, much to the chagrin of Hua Mulan, his young daughter, who feels that her father, who is ailing, should not be put through another campaign. Mulan would make a good soldier; she is smart, quick-thinking and skilled in martial arts, but as a woman is unable to enlist. That does not stop her; one night she sneaks out of the house, taking her father's armor and weapon with her. She disguises herself as a boy and joins the army in her father's place.
She becomes friends with the battalion's sub-commander, Wentai, as well as a couple of other recruits around her age. Training is rigorous and difficult but she proves that she is skilled in combat and has tremendous courage when she defends her fellow recruits from the commanding officer's nephew, Turtle, who is a bully.
Wentai discovers that there is a woman at camp when he sees Mulan taking a bath in a hot spring, but he does not realize that he has seen her. She also has to succumb to a strip-search with the rest of the battalion when Turtle drops a valuable jade pendant and Tiger kicks it into the lake to get back at Turtle for being such a bully; Turtle being Turtle, he immediately accuses the battalion of theft. Mulan doesn't want anyone to find out that she is a girl, or that she enlisted in her father's place, because that would bring incredible shame on her family, so she admits that it was she who stole the pendant. She is imprisoned and sentenced to be executed.
She finally admits to Wentai that she is a woman and asks him to make sure that her father does not find out about her execution. He agrees immediately and promises that he will burn her body himself so nobody will find out her secret. Before her execution, the Rourans attack the camp where her battalion are staying. Wentai releases Mulan, but she decides to stay and fight with her battalion instead of making her escape. She kills the Rouran's commanding officer, leading to her pardon, and promotion to sub-officer. During the attack the battalion's commander was killed by the Rouran, and now that he does not have his uncle to protect him, Turtle stops bullying the other soldiers. He even tries to make some friends.
Wentai and Mulan are both promoted to general because of their achievements during the war, but it has been a hard war for Mulan; many of her friends are killed before her eyes. Wentai and his men are ambushed and he instructs Mulan to stay behind to guard the supply train whilst he reinforces the front, but Mulan is scared for Wentai. She delegates guarding the supply train to Scholar and heads on after Wentai, but when she returns she finds that Scholar was ambushed as well and is dead; she feels as though his blood is on her hands and is devastated. Wentai exhorts her not to get emotional. Mulan wants to leave the army, but he reminds her that nobody in the army actually wants to be there; they are their to do their duty, as must she.
Things go from bad to worse for Mulan; Wentai is killed and she finds solace in drinking but when she finds out that his last words for her were that she should remain strong, she emotinally regroups and returns to her army life. She achieves more victories. There is another element to what is happening in that Wentai is actually still alive; he and Tiger made up the story of his death so that she would be able to refocus, and detatch herself from her emotions.
Under their new ruler, Prince Mendu, the Rouran redouble their attacks so that they can finally take possession of the mineral-rich lands of the Wei that they do not have themselves. Mulan comes up with a strategy that she feels will enable a Wei victory. She suggests that if her unit engages the Rourans at a predetermined location which will enable a larger Wei force to ambush them. Mulan's troops then fight strongly but there are many injuries including Mulan. A huge dust storm consumes everyone on the battlefield; when Mulan re-emerges from the dust she catches sight of Wentai and is overjoyed. She orders a retreat back into the canyon when she sees the Rouran reserves enter the battlefield, but Tiger is among the soldiers who are too injured to make their retreat.
Mulan's troops are stuck in the canyon; supplies run dry, but Wentai gives Mulan his own blood to drink for sustenance. The Rourans execute the injured Wei troops, including Tiger. Turtle is killed in an attempt to stop the executions. Mulan realizes that her troops have been betrayed by the Wei commander in chief because he is jealous of her skill and the way in which the troops respect her. She rallies her troops for a fight to the death but Wentai goes to the Rouran commanders and offers to trade his life for hers and the lives of her soldiers. Mendu takes him prisoner and surprisingly keeps to the deal, sending provisions in to Mulan and her troops.
Mulan adopts another disguise; she dresses as a Rouran woman and enters the Rouran camp in an attempt to save Wentai. She reveals that she is a woman when she meets with the Rouran princess; both want to stop the war and enjoy peace between their nations. Mendu announces that he and the Emperor have come to an agreement about Wentail's ransom, and as the camp erupts into celebration Mulan sneaks into Mendu's tent and assassinates him with the help of the Rouran princess. She returns to the Wei capital and finally admits that she is a woman. The Wei Emperor knows that he owes the peace of his country to her and offers her anything that she desires as a reward, but all that Mulan wants is to be allowed to return home to her family as her father is ill and she would like to take care of him He grants her this wish, and then announces that Wentai and the Rouran Princess are to be married, which will bring peace and harmony between their nations for generations to come.
Wentai does not love his princess; he loves Mulan and visits her at her home to ask her to run away with him. As much as she loves him too, Mulan refuses because she feels a sense of duty and chooses the good of the country over the desires of her heart. Wentai respects her decision but they share a passionate kiss before he leaves again.
