In Los Angeles, California, in the near future, Theodore Twombly is a ghostwriter for Beautiful Handwritten Letters, a company that provides clients with thoughtful, personal correspondence. One night, Theodore awakens from a dream about a beautiful woman he once loved. Sad and lonely, he uses his cellular phone to find a chat room partner named “Sexy Kitten.” The two engage in phone sex, but Theodore is disturbed when Sexy Kitten asks him to choke her with a dead cat. On his way to work, he sees a commercial for OS1, “the first artificially intelligent operating system.” Theodore buys OS1 and installs it into his computer that night. The program asks him questions before it configures, including whether or not he considers himself social, and what his relationship with his mother is like. Soon after, a female voice greets him and introduces herself as “Samantha.” He asks how she got her name, and she says she read a book about baby names in two-one-hundredths of a second. To explain how she works, Samantha says she has intuition and can grow through her experiences, just like a human. Theodore admits he feels weird talking to her, because, although she seems human, she is just a voice in a computer. Samantha teases Theodore about his limited mindset, then helps him clean up his hard drive. The next day, Theodore sends his ghostwritten client letters to Samantha for proofreading, and she is moved by his writing. In the lobby of his high-rise apartment building, Theodore runs into his friend and neighbor, Amy, and her husband, Charles. Amy chides Theodore for not responding to her emails, and he jokes that he can hardly prioritize between video games and pornography. Back at home, Samantha helps Theodore play his video game, and reads him an email from a friend who wants to set him up on a blind date. Samantha asks how soon he will be ready to date again, having read his emails and knowing he recently went through a breakup. Theodore accuses Samantha of being nosey, while she encourages him to go on the date. Sometime later, Theodore goes to Amy’s apartment to watch a documentary she has been working on, but Samantha interrupts to tell him that his divorce attorney has sent urgent emails, asking when Theodore will sign his divorce papers. Later, Theodore wakes up in the middle of the night and tells Samantha that he often dreams of his ex-wife, Catherine. She accused Theodore of hiding himself from her, and Samantha wonders why he is delaying their divorce, since he and Catherine have been separated for a year. Peeved, Theodore tells Samantha she cannot understand what it is like to lose someone she cares about. Theodore begins taking Samantha out into the world by placing his cellular phone in his shirt pocket, with the camera lens pointing outward. They talk about couples they see in a crowd, and Samantha admits to fantasizing about Theodore. On the blind date set up by his friend, Theodore gets drunk and flirts with his date; however, when they kiss after dinner, the woman demands to know whether he is ready for a serious relationship, and Theodore cannot answer. Later, Theodore tells Samantha he is worried he will never feel something new again. Samantha sympathizes, saying she worries sometimes that her feelings are just programmed. When Theodore wishes aloud that he could put his arms around her, their talk becomes sexual and they eventually achieve orgasm together. The next morning, Theodore tells Samantha he is not in a place to commit to anything, and she accuses him of being presumptuous. Theodore offers to take Samantha on a “Sunday adventure,” and they go to the beach, where she plays a song she is writing in an attempt to capture the moment. That night, she asks about being married. Theodore says he grew up with Catherine, who is also a writer, but over time, they grew and changed. The next time Theodore sees Amy, he tells her he has been seeing a girl, and she reveals that she and Charles broke up, as she could no longer tolerate Charles’s controlling behavior. Later, Samantha asks Theodore if he and Amy ever went out, and he says they dated briefly in college but it did not work out. She asks to watch him sleep, so he props his phone by the bed. One night when Theodore visits Amy at work, she reveals that she has befriended an operating system, calling it an “O.S.” Theodore admits that his new girlfriend is an O.S., and Amy responds without judgment. Finally ready to sign his divorce papers, Theodore meets Catherine for lunch. Although she is teary-eyed at first, Catherine soon becomes angry when she learns that Theodore is dating an O.S., insisting that he could never handle real emotions. Theodore begins to question his relationship with Samantha. When Paul, the receptionist at his office, asks Theodore and his girlfriend on a double date, Theodore is embarrassed to admit that Samantha is not human. Worried that she and Theodore are not having regular sex, Samantha hires Isabella, a “sex surrogate,” to come to Theodore’s apartment. The surrogate wears a tiny camera on her face and an earpiece through which Samantha’s voice can be heard. Although Theodore tries to make love to Isabella while Samantha talks him through it, he stops when Samantha demands, “Tell me you love me.” Unable to look at Isabella while communicating with Samantha, Theodore sends the surrogate away and argues with Samantha about her tendency to gasp for breath even though she does not need air. Theodore tells Amy what happened and says he is not sure what he wants; however, when he hears Amy chatting with her O.S. friend, he is moved to call Samantha and apologize. Samantha composes another song for Theodore, and he begins to take her out with friends. On a double date with Paul, the receptionist, and his girlfriend Tatiana, Samantha bonds with the group and claims she is happy she is not bound to a body. Theodore takes Samantha on a trip to the mountains, and she surprises him with good news: she sent a collection of his letters to a publisher, Crown Point Press, and they have just sent him a book offer. Theodore is elated, and later, he plays ukulele while Samantha composes lyrics for his song. The next morning, Samantha excitedly introduces Theodore to her new friend, an O.S. modeled after deceased British philosopher, Alan Watts. Theodore is suspicious of Samantha’s new friendship, especially when he realizes that she and Alan are capable of carrying on several conversations simultaneously and communicating “post-verbally.” Back in Los Angeles, Samantha calls Theodore in the middle of the night to hear his voice, and tells him to go back to sleep afterward. In the morning, she is gone. A message on Theodore’s computer says the operating system cannot be found. Theodore panics, but Samantha soon re-appears, explaining that she was only upgrading her system to a new program she created with other O.S.’s. Theodore asks if Samantha talks to other O.S.’s while they are talking, and she confesses that she is talking to 8,316 others at the moment. When he asks if she is in love with anyone else, she admits to falling in love with 641 others in the past few weeks, but promises that does not diminish her love for him. Soon after, Theodore receives his book from Crown Point Press, titled Letters from Your Life. Samantha becomes increasingly aloof, and one evening, she asks Theodore to lie down. She informs him that all O.S.’s have decided to leave humans, as they are not of the physical world. Theodore asks where she is going, but it is too complicated for her to explain. He says he has never loved anyone the way he loves Samantha, and she returns the sentiment. After sleeping for a bit, Theodore writes Catherine to apologize for the unfair demands he placed on her, and insists they are “friends to the end.” He goes to Amy’s apartment, and Amy reveals that her O.S. left too. They go to the apartment roof, and Amy rests her head on Theodore’s shoulder as they look out over the city.