Sixteen-year-old Ella is genetically engineered and meticulously trained to be a pet, an obedient companion to the wealthy families who can afford the luxury. A pet’s sole purpose is to enrich the lives of her owners through music and the arts, and to provide happiness to others but never to themselves.
The kennel, the place where pets are bred and groomed, holds an exclusive, private sale. While several buyers consider the 20 pets that are offered, Ella is sold to Congressman John Kimball and his wife, Elise.
Congressman Kimball pushed for the legalization of the creation and sale of pets, so he feels that he should own one. Ella notices that Elise is unhappy about her husband’s decision to purchase a pet, but she relents when John reminds her that they need a companion for their 10-year-old daughter, Ruby.
At first, Ella is thrilled to be bought by the Kimballs and with the beauty of their elaborate home. She is also happy when she meets their teenage son, Penn, as he is the first boy close to her own age she’s ever met. Like his mother, Penn is cold toward Ella.
Ella learns that she is the Kimballs’ second pet. The first pet became sick and was returned to the kennel for special care. This information scares Ella as her trainers have told her imperfect pets are eliminated. Pets who are sent back to the kennel never return.
As Ella settles into her new life — filled with beautiful gowns and days spent playing with Ruby — she begins to notice her limitations. She has been taught how to play a piano and sing flawlessly, but she doesn’t know how to read, write or swim. She also doesn’t have any knowledge of the world outside the kennel and the mansion.
The Kimballs’ neighbor Ms. Harper comes to the house and gets into a huge argument with Elise. Ms. Harper is furious that the Kimballs have bought a second pet and accuses the family of being slavers. Ella realizes that not everyone supports the practice of owning pets.
A few days later as Ella is exploring the Kimballs’ property, she sees Ms. Harper again. The woman apologizes for the outburst and offers to take her to safety, rescuing her from slavery. Ella refuses to leave her pampered life.
Ella is attracted to Penn, and when the Kimballs throw an elaborate party, she sneaks away from the main festivities to the garden with him. The two dance, and he shows her an unmaintained portion of the garden that only he visits. The garden has a pool, and he promises to teach her how to swim. The two are interrupted when Elise finds them, telling them that John is looking for Ella so he can show her off to his friends.
Ella is put on display for adult men, and they ogle her. When she gets a chance, Ella breaks away only to be propositioned by a young man, whom she promises to meet inside the house. As soon as the boy leaves, Ella runs as far as she can to the edge of the property where she sees Ms. Harper.
Tricking Ella, Ms. Harper pushes Ella into her car and kidnaps her, driving her to a safe house and leaving her there. The owners of the safe house refuse to help Ella without being paid, so they turn her out. Unfamiliar with the outside world and unable to function, Ella hides in a park.
The police use the tracking device that’s been implanted behind her ear to find her. They call the congressman to pick her up. Even though Ella tells the Kimballs that she was kidnapped, they are suspicious of her, thinking she may have tried to run away. John is more watchful of her.
Ella and Penn, with their shared passion for music, grow even closer. When the rest of the family isn’t home, they spend time in Penn’s room listening to jazz. At night, they sneak out of the house to the hidden garden pool where Penn teaches Ella how to swim.
While out for a walk with Ella, Ruby runs into one of her classmates, Jayne, who invites the girls back to her house so they can play dress up with their pets. Jayne’s pet, Missy, is older than Ella. At first, Ella hopes to become friends with Missy, but she soon realizes that Missy is jaded and cruel. When the pets are alone, Missy tells Ella that her perfect life with the Kimballs won’t last forever, that when she gets older, they will tire of her and sell her off to the highest bidder.
The Kimballs’ eldest daughter, Claire, and her fiancé come home for a visit. Claire does not like Ella and is openly resentful toward her. During Claire’s visit, John surprises Ella by taking her to the beach. As she stares in awe at the ocean, John whispers to her that she should come to him for swimming lessons instead of Penn. He tells her to stay away from his son, as a relationship between them is impossible.
When she gets home, she overhears a conversation between Penn and Claire. Claire reminds Penn that Ella belongs to their father, how the other pet ruined their family and that Ella is not a person, just a pet. Penn tells Claire that he knows Ella is just a pet and that he is not afraid of his father’s anger, even though Claire assures him that he should be. Ella stumbles away from the conversation and in a moment of hurt and panic, runs to Missy’s house, hoping she can confide in the other pet.
Climbing through Missy’s window, Ella tells the older girl that she didn’t mean to fall in love with Penn and that she is scared to go back to the Kimballs’ house, for his safety. Ella tries to convince Missy to run away with her, but Missy tells her that no one will help them and that it’s not worth the risk. Missy met girls who have been sold two and three times to horrible homes and forced to do things for their owners. She prefers to live the few good years she has in comfort than face that future. Missy apologizes to Ella before screaming for help and lying to her owner that Ella tried to kidnap her.
Ella is caught, and John is furious when he comes to pick her up. He takes her home and puts shackles on her wrist and chains her to a post in her room, preventing her from running away again. At breakfast the next morning, Ella (now chained to the wall so she can eat), finds out that John is taking her back to the kennel to be spayed — in the hopes that sterilizing her will get her hormones under control and discourage her from running away again. Ella is horrified.
Penn is disappointed that Ella would run away without telling him, and he sneaks into her room late that night to talk. Penn tells her that they have to find a way to get her to Canada where Ella will receive asylum. Discouraged, Ella tells him that escape may not be worth it, but Penn reveals that their last pet wasn’t sent back because she was sick, she was pregnant. John told the family that a gardener was responsible, but no one in the family truly believed him.
Ella is scared of what John will do to Penn if he helps her escape, and she is frightened to go to Canada, as she doesn’t know how to function on her own. Penn promises to go with her, leaving his family and life behind.
Penn gets bolt cutters and cuts the shackles from Ella’s wrist, and Ella uses a razor to cut out the microchip behind her ear. Before they can get out of the house, Elise confronts them. Instead of stopping them, she gives Penn money and a Canadian address to help with their escape.
Penn and Ella drive to Niagara Falls, hoping to enter Canada at the border, but Penn knows his father will guess where they are heading. As they near the crossing, they see the police on the American side checking the cars. Penn ditches the car, and they slip into a car rental agency, hoping to quickly rent another. The attendant recognizes Ella from a reward poster and calls the police.
Penn and Ella run and are helped by a married couple who also recognize Ella from the reward poster. They attempt to get Ella and Penn across the border, hoping that the police won’t be suspicious of a car with four people. The first police officer lets them pass, but soon, the other officers who saw them get into the car pursue them across the bridge. Ella and Penn get out of the car and run for the Canadian border. Penn promises Ella he is right behind her but he gets caught, all the while yelling at her to run.
She runs to the Canadian officers, begging them for asylum while the American police demand she be returned as the property of Congressman John Kimball. The Canadian officer tells the American that she isn’t property and will not be returned. Penn, still on the American side, is arrested and taken away, and the Canadian officers stop Ella from going back to him. Ella only has the Canadian address given to them by Elise. Even though she doesn’t know how she will make it, she moves forward, tasting freedom for the first time.