Plot
Calvin Weir-Fields is a novelist who found incredible success at an early age, but struggles to form relationships and to write his next book. His therapist tasks him to write about someone who likes his dog, Scotty. After a dream in which he meets a woman, Calvin is inspired to write about her, admitting that he is falling in love with his character, "Ruby Sparks".
Calvin discovers a woman's belongings throughout his house, to the surprise of his brother Harry and sister-in-law Susie. Harry criticizes Calvin's writing about Ruby as overly idealized and unrealistic, but Calvin writes a passage about Ruby falling in love with him before falling asleep at his typewriter.
The next day, he is stunned to find Ruby in his kitchen, a living person, who believes they are dating. He calls Harry, who does not believe him and advises him to find someone to confirm she is not real. Ruby insists on coming along as Calvin meets with Mabel, a fan of his book who gave him her number. Believing Calvin is cheating on her, Ruby confronts them, proving she is not a figment of his imagination.
Calvin throws himself into their relationship, with Ruby unaware that he wrote her into existence. He introduces her to an incredulous Harry, demonstrating that his writing directly affects her, and asks Harry not to tell anyone of Ruby's origins, declaring that he will never write about her again.
Months later, Calvin reluctantly takes Ruby to meet his free-spirited mother Gertrude and her boyfriend Mort. While the outgoing Ruby enjoys herself, the introverted Calvin grows jealous of her time with other people, and her happiness fades with his increased gloominess.
Returning home, their relationship becomes tense and a depressed Ruby explains how lonely she is, suggesting they spend less time together. Fearful of Ruby's desertion, Calvin writes that she is miserable without him and she returns, now incredibly clingy. Tiring of this, he writes that she is constantly happy but he becomes morose, knowing her happiness is artificial.
Confiding in Harry, Calvin tries to write Ruby back to her original self, but his wording leaves her confused. They fight once more, and he attempts to cheer her up by taking her to a party hosted by his mentor, author Langdon Tharp. Leaving Ruby on her own, Calvin runs into his ex-girlfriend Lila, who accuses him of being uninterested in anyone outside of himself. Langdon flirts with Ruby, convincing her to join him in the pool in her underwear, but a furious Calvin drives Ruby home.
They fight, and Ruby tells Calvin that he cannot control her. As she prepares to leave, Calvin reveals that she is a product of his imagination and that he can make her do anything he writes. Their growing argument leads to Calvin forcing Ruby to perform increasingly frenzied and humiliating acts. She collapses, and when Calvin tries to approach her, she locks herself in his room.
Distraught and ashamed, Calvin writes a final page stating that as soon as Ruby leaves the house, she is no longer his creation, no longer subject to his will, and is free. He leaves the manuscript outside her door with a note telling her to read the last page and that he loves her. The next morning, Calvin finds the note and Ruby gone.
Time passes, and Harry suggests Calvin write a new book about his experiences with Ruby. The novel, The Girlfriend, is a success. Walking Scotty in the park, Calvin sees a woman who appears to be Ruby but has no recollection of him, reading his new book. She says that Calvin also seems familiar, which he deflects by showing her his author's photo. She jokes that they ought to start over, urging him, "Don't tell me how it ends," to which he replies, "I promise."
