In Ruby Sparks Calvin Weir-Fields (Paul Dano) is a young novelist, struggling to recreate the early success of his first novel, but unable to commit to any of his ideas. His therapist, Dr. Rosenthal (Elliott Gould), gives him a writing assignment: to write a page about someone who likes his dog, Scotty. Calvin has a dream in which he meets an attractive young woman, who draws a picture of Scotty and says she likes him. Calvin wakes and is inspired to write about her. In therapy he admits he is falling in love with the character he is writing, and tells the therapist all about Ruby Sparks (Zoe Kazan).
Calvin's brother Harry (Chris Messina) and sister-in-law Susie (Toni Trucks) come to visit, and find several articles of women's clothing around the house. The next day, Calvin is stunned to find Ruby in his kitchen, as an actual living person. Thinking he is going crazy, he calls Harry, who does not believe him and advises him to meet with someone else to take his mind off things. Ruby is confused by his behavior and insists on coming along, and leaves her to shop while he meets Mabel (Alia Shawkat), a fan of his book. Ruby finds them, and Calvin discovers that others can actually see Ruby, discovering that she is a real person, not a figment of his imagination. Ruby fights with Calvin, thinking he is cheating on her but he explains how he feels overwhelmed and they make up.
Calvin(Ruby Sparks) introduces Ruby to Harry, and Harry believes there must a be logical explanation, but Calvin proves that his writing directly affects Ruby. Harry is amazed that Calvin has manifested a woman with his mind, and has the power to change her. Calvin says he loves Ruby and asks Harry not to tell anyone of Ruby's origins. Harry warns him that women are mysterious creatures, and that things may change. Calvin says he wrote her and he knows her, and he will never write about Ruby again. Months later, Calvin reluctantly takes Ruby to meet his free-spirited mother Gertrude (Annette Bening) and boyfriend Mort (Antonio Banderas). While Ruby and his family enjoy themselves, Calvin spends the weekend reading by himself, growing jealous of the time Ruby is spending with other people and Ruby's happy spirit begins to dampen at Calvin's increased gloominess.
Back at Calvin's house(Ruby Sparks), Calvin complains to Ruby that her singing is disturbing his reading. Upset, she runs upstairs crying. Ruby tells Calvin how lonely she is and suggests they start spending less time together. Calvin is miserable without her and, out of desperation, writes that Ruby is also miserable without him. Ruby returns to Calvin's house full-time, but becomes incredibly clingy toward him, afraid to leave his side for even a second. Calvin tires of this and writes that Ruby is instead "filled with effervescent joy," at which point she becomes constantly happy.
After talking with Harry(Ruby Sparks) about what he has been doing, Calvin intends to write Ruby back to her normal self but the wording he uses leaves Ruby confused and slightly manic-depressive. Gloomy again, Ruby fights with Calvin once more. He attempts to cheer her up, and takes her along to a party hosted by author Langdon Tharp (Steve Coogan). At the party, Calvin leaves Ruby and talks with people about his still-unfinished manuscript. He also runs into his ex-girlfriend Lila (Deborah Ann Woll), and they have a heated argument in which Lila accuses Calvin of only seeing her in an idealized way. Langdon sees Ruby alone and flirts with her, convincing her to strip to her underwear and join him in the pool. Calvin finds Ruby about to join Langdon for a swim and they leave.
At home, Calvin and Ruby(Ruby Sparks) fight, with Ruby accusing Calvin of controlling of her. As she prepares to leave Calvin, he reveals that she is a product of his imagination, and that he is capable of making her do anything he writes. He demonstrates this to her making her perform various actions. Eventually Calvin stops, his head bent in anguish over his typewriter; while Ruby, exhausted, runs into his bedroom and locks the door.
Calvin writes a final page, which states that Ruby is no longer his creation, subject to his will, and she is free. He leaves the manuscript outside, with a note. The next morning, Calvin finds that the note is gone along with Ruby and he breaks down sobbing. Harry suggests he write a new book about his experiences with Ruby. The novel, The Girlfriend is a success. While walking Scotty in the park Calvin sees Ruby, who has no recollection of him. She is reading his book, which she says her friend described as pretentious. Calvin reveals himself as the book's author, and she asks to start over, and they sit and talk. She asks him not to tell her how the story ends of Ruby Sparks.
Gener: Comedy, Fantasy