Calvin Weir-Fields (PAUL DANO) is a novelist whose first work -- written in high school ten years ago -- was a runaway success and labeled him as America's next great author. Since then, however, he's been unable to match that work and often finds himself afflicted with writer's block. That's much to the chagrin of his agent, Cyrus (AASIF MANDVI), who can get him, however reluctantly, to share the stage with fellow novelist Langdon Tharp (STEVE COOGAN) and attend book signings for his fans.
To make matters worse, the only women Calvin seems to attract are those interested in him only due to his fame, a point not lost on Calvin's brother, Harry (CHRIS MESSINA), who's married to Susie (TONI TRUCKS). Harry even laments that Calvin can't get women in his dreams to go to bed with him, just one of various matters that the author discusses with his therapist, Dr. Rosenthal (ELLIOT GOULD). The latter eventually convinces Calvin to write just one bad page to get the ball rolling again.
Inspired by another visit by his ideal dream girl, however, Calvin does more than that, and starts cranking out the pages about Ruby Sparks, his latest literary creation. Little does he realize that he's also somehow magically created her in person as a flesh and blood woman (ZOE KAZAN) who sees herself as his girlfriend and is unaware of her unusual genesis.
Calvin initially thinks he's lost his mind, a sentiment shared by Harry, but both not only soon accept that she is indeed real, but also that whatever Calvin types about her comes true, such as suddenly making her speak French. With her unaware of being manipulated, Calvin finds himself content with his girlfriend, even if she wants to meet his hippie style mother, Gertrude (ANNETTE BENING), and her artist boyfriend, Mort (ANTONIO BANDERAS), who live up the coast from Los Angeles.
Determined not to write anything more about Ruby, Calvin starts to have a change of heart as their relationship develops and she soon starts wanting some more personal space. From that point on, Calvin must contend with the ramifications of changes that he makes to her behavior that end up affecting their relationship as a couple.