In 2007, "environmentalist" Lucy Mirando becomes CEO of the Mirando Corporation, succeeding her twin sister. Announcing they have been breeding a special kind of "super pig", 26 specimens are sent to farmers around the world, and ten years later, one will be crowned the winner as the best pig.
In 2017, a young girl named Mija lives in South Korea with her grandfather and their super pig, Okja. Mija and Okja have a very close relationship; they spend much of their time together, and at one point, Okja saves Mija from falling off a cliff. They are visited by Mirando spokesperson and zoologist Dr. Johnny Wilcox, who declares Okja the best super pig, and announces that they will take her to New York City. Her grandfather gives Mija a solid gold pig figurine, explaining he saved up to replace Okja when she was taken away. Devastated, Mija goes to Seoul to find Okja, where she sees her being loaded onto a truck. Mija chases down the truck, but it is intercepted by the Animal Liberation Front.
In the resulting chaos, Mija and Okja run away but are eventually saved by the ALF, led by Jay. He asks another ALF member, K, to translate and tell Mija that they plan to put a recording device in Okja's ear and let her be recaptured by the Mirando Corporation to document how they mistreat animals. Mija asks them to return her to the mountains, but K purposely mistranslates and tells them Mija agrees. They leave, and Okja is recaptured.
To minimize PR damage to the company, Lucy pays for Mija to come to New York to stage a heart-warming reunion with her pig. Okja is taken to a laboratory where she is forcibly bred with another super pig, and flesh is taken from her for a taste test. After the ALF sees the footage, K reveals that he lied to the rest of the group about Mija's support of the plan. Jay beats K, and expels him from the ALF.
In New York, Mija has to comply with the Mirando Corp. Jay slips into her room and tells her they plan to rescue Okja while on stage. During the Mirando parade, a battered and temporarily blinded Okja attacks Mija. Jay tries to hurt Okja to protect the girl, but Mija doesn’t let him and calms Okja down. The ALF shows Okja's mistreatment to the public, who quickly turn against Mirando. Lucy surrenders the company to her twin sister Nancy, who contacts a private security firm Black Chalk to take out the ALF members. Okja is recaptured and the ALF members are arrested, except for Mija and Jay, who are rescued by a remorseful K. Nancy starts full production at the slaughterhouse.
K, Mija, and Jay travel to a processing plant in search of Okja, and find her being forced up a ramp into a slaughterhouse. On the verge of being slaughtered, Mija shows a Mirando employee a photo of herself with baby Okja, prompting him to stop. Nancy arrives and Mija offers the gold pig in exchange for Okja's life. Although initially reluctant, Nancy agrees, as she deemed it a good business deal, having Jay and K arrested. As Mija and Okja are escorted away, a pair of super pigs push their newborn through the electric fence to Okja to hide and rescue from the farm.
Back in the countryside, Mija resumes her life with her grandfather, Okja, and the new piglet.
In a post-credits scene, Jay is released from prison, boarding a bus with K and the other members of the organization. With their newest member Kim Woo-shik, a former driver for Mirando, they plan to disrupt a major Mirando shareholders meeting.
