Set in an alternate England of the early 1800s, the film opens with Colonel Darcy (Sam Riley) traveling to the home of a wealthy family to investigate the emergence of a zombie. He arrives at the home and sits with the family before taking out a small vial containing a particular set of flies that can detect the undead. The flies land on the house's patriarch, whose eyes turn red. Darcy breaks a glass and stabs the man in the throat before decapitating him. This frightens one of the young girls in the house, and she retreats upstairs. When she is discovered, she has turned into a zombie as well and proceeds to attack.
Over the opening credits, we hear the voice of Mr. Bennet (Charles Dance) narrate the origin of the zombie outbreak, which is represented in the form of a pop-up book. The Black Plague hit London and began to create zombies. A wall was built to separate London from the zombies, leaving them in an area called The In-Between. It is said that the Four Horsemen are ready to rise up from Hell and bring about the apocalypse.
The five Bennet sisters: Elizabeth (Lily James), Jane (Bella Heathcoate), Kitty (Suki Waterhouse), Lydia (Ellie Bamber), and Mary (Millie Brady) have all been trained by their father in the art of weaponry and martial arts after being sent abroad to China. Mrs. Bennet (Sally Phillips) wants her daughters to be married off to wealthy suitors. As it turns out, the Bingley family has moved in nearby and are throwing a ball, wherein Mrs. Bennet hopes that the young and handsome Mr. Bingley (Douglas Booth) will win over one of her girls. Elizabeth doesn't want to seek out a husband, to which Lydia makes a snide comment about Elizabeth not being able to attract men. She proceeds to chase Lydia out of the room.
The Bennets attend the ball. Bingley instantly sets his eyes on Jane. Darcy is in attendance and takes notice of Elizabeth, but he makes comments that she is unappealing, which Elizabeth overhears. She walks outside tearfully and encounters a woman she knew named Mrs. Featherstone (Hermione Corfield), who is a zombie. Mrs. Featherstone is about to tell Elizabeth something until her head is blown off by Darcy. A horde of zombies then attacks the party, prompting the Bennet sisters to spring into action. As they slay every zombie in their path, Darcy instantly becomes smitten with Elizabeth when he witnesses her in combat.
Jane is outside by herself on a rainy day and encounters a zombie. She kills it but then spots a zombie woman with her child. Jane hesitates and is attacked by the zombie. She is confined to her room, where it is feared that she may have been bitten. Darcy arrives at their home with the flies to detect a zombie, but Elizabeth catches each fly with her hand and returns them to Darcy.
After Jane recovers, the sisters attend another ball for the Bingleys. There, Elizabeth meets the soldier Mr. George Wickham (Jack Huston), who seems to be charming and polite. He tells Elizabeth that he has had a history with Darcy and does not wish to further challenge him. Another group of zombies attacks the party, resulting in Bingley injuring himself (but not getting bitten), and Darcy joins Elizabeth in fighting the monsters.
The Bennets are visited by Parson Collins (Matt Smith), who intends to marry one of the sisters. He initially sets his eyes on Jane, though he is told that she is with Bingley. He later tries to make a move on Elizabeth, but requests that she give up her life as a warrior, something that she adamantly refuses to do, even for a man. Mr. Collins later decides to settle with Elizabeth's friend Charlotte (Aisling Loftus).
Elizabeth and Darcy meet with Darcy's aunt, Lady Catherine de Bourgh (Lena Headey), a notorious zombie killer with major authority. Lady Catherine, who has several scars on her body that she carefully hides as well as a black eyepatch over her left eye, wants Darcy to marry her daughter Anne, but Catherine's skill are also needed to fight off the zombies.
Elizabeth travels with Mr. Wickham to the In-Between, an area outside of walled-in London but inside a royal moat, to a church filled with zombies, who feed on pig brains to keep themselves from going completely savage. Mr. Wickham wants Elizabeth to join him in helping the zombies. He also suggests that she run away with him, but she remains conflicted.
Back home, Elizabeth is told that Darcy has convinced the Bingleys to move away. When Darcy approaches Elizabeth with a proposal of his own, she expresses her outrage at his actions and fights him in a duel. Elizabeth gets the upper hand, and Darcy leaves.
Mr. Darcy writes Elizabeth a letter to apologize for his actions and to state that he separated Jane and Mr. Bingley for fear that Jane only wanted to marry Mr. Bingley for his wealth, having overheard Mrs. Bennet drunkenly mention it. Mr. Darcy also mentions that Mr. Wickham had tried to elope with Darcy's 15-year-old sister for her fortune. Mr. Darcy's letter states that he is battling zombies in London, and that they have overrun the walled city. Elizabeth is then cornered by Lady Catherine and her bodyguard Wilhelm (Ryan Oliva). Lady Catherine states Mr. Darcy has been intended to marry her sickly daughter, Anne, from their youth and confronts Elizabeth about rumors over Elizabeth and Mr. Darcy. When Elizabeth denies it, Lady Catherine instigates a fight between Elizabeth and Wilhelm in place of Lady Catherine who Elizabeth has refused to fight. Wilhelm attacks Elizabeth, but she claws his face and fights him off, causing him to get crushed under a pile of bricks. Catherine then holds her sward at Elizabeth's neck and states she doesn't know if she admires Elizabeth's skill as a warrior, or her resolve as a woman. She leaves Elizabeth alone.
Afterwards, Lady Catherine decides to protect Elizabeth's family from approaching zombies and takes them to her estate. Elizabeth later finds out that Mr. Wickham ran off with her younger sister, Lydia and decides to go rescue her.
Elizabeth and Jane travel to the canal on the outskirts of London where the Royal Army is blocking the last bridge out of the city. Elizabeth the leaves Jane behind in the army camp and hides her horse alone across the bridge to find her sister.
Meanwhile, the zombies make their way toward London. Mr. Darcy then encounters Mr. Wickham at the old church and rescues Lydia in the basement. When a horde of zombies, who had been living normally until the pig brains they had been eating were switched with human brains by Mr. Darcy storm the cellar, Mr. Darcy escapes with Lydia.
Elizabeth rejoins Darcy in the In-Between and helps him battle the undead. Darcy encounters again Wickham and fights him, eventually impaling him and seeing a bite mark on his chest, revealing he was undead all along. Before Wickham can kill Darcy, Elizabeth rides in and chops his right arm off and knocks him out. Darcy rides with Elizabeth across the bridge as other Londoners blow it up to keep the zombies away. Darcy is injured in the explosion and is rendered unconscious. Elizabeth tearfully admits her love for him.
After Darcy recovers, he finds Elizabeth and tells her that he heard what she said on the bridge. They share their first kiss and agree to marry. The Elizabeth and Darcy have a joint wedding with Bingley and Jane, officiated by Parson Collins. Everyone attends including Lady Catherine, Mr. and Mrs. Bennett, as well as Lydia, Kitty and Mary. The two couples walk away together to live happily ever after...
Until a moment during mid-credits when everyone sees something beyond the church gates. The now one-armed Wickham is leading the four zombie Horsemen along with a HUGE army of zombies towards them, ready for war....