Calvin Weir-Fields, a talented writer struggling with personal connections, receives an unusual recommendation from his therapist: to craft a narrative centered around someone who shares a deep bond with his beloved dog, Scotty. This unexpected suggestion ignites a vivid dream in which he conjures a remarkable woman named Ruby Sparks. Captivated, Calvin finds his fingers flying over the typewriter, unleashing the story of Ruby as if she were a real person.
Suddenly, Calvin discovers Ruby's belongings mysteriously unpacked in his home, leaving his skeptical brother, Harry, and sister-in-law, Susie, utterly perplexed. Harry dismisses the fantastical elements of Calvin's writing, criticizing it for being unrealistically idealized. However, Calvin remains steadfast, determined to expand his creation beyond the realms of imagination.
One evening, after tiring of writing, Calvin drifts into sleep with Ruby's tale still fresh in his mind. To his astonishment, he awakens the next day to find Ruby Sparks herself—played by Zoe Kazan—alive and eager to embark on a romantic journey with him.
As Calvin's fantasy unfolds into his reality, he grapples with an intense emotional dilemma; while his affection for Ruby deepens, he struggles with the immense responsibility of having brought her to life. To convince Harry of her existence, Calvin invites Ruby to meet Mabel, a devoted admirer of his work. Their encounter leads to confusion, as Ruby reveals her profound reality in a world that seems to blur the lines of creation.
With Ruby now an integral part of his life, Calvin introduces her to his incredulous brother and mother, Gertrude, who is navigating her own love life with the charming Mort. However, as Ruby’s vibrancy wanes, so too does Calvin’s happiness, creating a perilous strain on their relationship.
To combat this decline, Ruby suggests they create some distance, but Calvin's fear of losing her grows unbearable. In an attempt to solidify her love, he writes that Ruby is unhappy without him, leading her back into his arms. But this dance of control takes its toll; Calvin’s efforts to manipulate Ruby's emotions reveal deeper issues within himself.
As they navigate this surreal partnership, the tension escalates. At a party hosted by his mentor, Langdon Tharp, Calvin’s insecurity heightens when he spots his ex-girlfriend Lila, leading Ruby to accuse him of emotional unavailability. The pressure of Langdon's flirtations further exacerbates Ruby's frustrations, spiraling their relationship into chaotic territory.
Calvin’s desperate need to control Ruby culminates in a series of absurd demands that push her to her limits, ultimately crushing her spirit. When she retreats to his room in despair after a particularly harrowing scene, Calvin's ensuing guilt drives him to write one last passage in his manuscript, granting Ruby the freedom to leave him. He lovingly leaves a note outside her door, but she has vanished, leaving an aching void behind.
Months pass, and Harry advises Calvin to pen another book based on his tumultuous experience with Ruby. Titled The Girlfriend, it skyrockets to popularity, marking a new chapter in his career. One day, while strolling through the park with Scotty, Calvin spots a woman reminiscent of Ruby, lost in reading his latest book. Their eyes lock, and she playfully brings up their shared past, making Calvin flash his author photo to avoid further entanglement. She teasingly suggests they take a chance on a fresh beginning, and Calvin responds with an earnest vow: > "I promise."
