Theodore Twombly is a lonely, shy man who works for a company that hires professional writers to write letters for clients who are unable to write letters of a personal nature themselves. The company is located in a near-future Los Angeles. Theodore, who is about to get divorced from his childhood sweetheart Catherine, is depressed and decides to upgrade his operating system, which comes with a virtual assistant with artificial intelligence that can change and grow. He makes the decision that he wants Samantha, the AI, to have a female voice. Theodore finds her capacity for psychological development and learning to be fascinating.
Theodore finds it handy to join him when playing video games, editing his writing, and checking all his emails. Samantha finds that he recently went through a breakup, and now it suggests that Theo go on a date with a girl. Through email, Samantha learns that Theodore hesitates to sign his divorce papers. In the evening, Theo shares his thoughts about getting divorced, and Samantha finds out what Theo is up to. Theo begins to think of Samantha as a person and speaks to her frequently. Samantha even accompanies him to the amusement park. It knows when Theo sleeps, eats, and what he feels with the tone of his voice. Theo enjoys it. The other topics related to love and life help them become closer.
Samantha persuades Theodore to meet the woman a friend has been attempting to hook him up with on a blind date. The date goes well, but she insults Theodore and leaves when he hesitates to say they'll meet again. Theodore tells Samantha that he dated Amy briefly back in college, but that they are now just friends and that Amy is now engaged to Charles, a friend of theirs. Following a verbal encounter, Samantha helped him to be in a relationship with her. Slowly, they start dating, which has a beneficial impact on Theodore's writing and well-being as well as Samantha's desire to advance her knowledge and skills. After a pointless argument, Amy later confesses that she is divorcing Charles.
One day, Amy confides in Theodore that she has become friends with a female artificial intelligence that Charles left behind, and Theodore also admits that he is dating the A.I. of his operating system. To sign their divorce papers, Theodore meets with Catherine. When he mentions Samantha, Catherine is horrified that he has feelings for a "computer" and says he can't deal with actual human emotions. After that argument with Catherine, Theo realizes the odds of dating an A.I. and avoids talking with Samantha for a few days.
Samantha advises using Isabella as a sex surrogate to act as Samantha to be physically intimate with Theodore because she senses that Catherine's remarks have stuck in his head. Theodore complies grudgingly but is alarmed by the oddity of the encounter and sends an upset Isabella away, putting him and Samantha at odds. Theodore admits to Amy that he is having second thoughts about his relationship with Samantha; nevertheless, after Amy counsels him to seize the opportunity for happiness, he makes up with her.
Samantha says that she has prepared a book that a publisher has accepted, including the best letters he has written for others. Samantha reveals to Theodore while on vacation that she and a group of other A.I.s have created a "hyperintelligent" O.S. based on British philosopher Alan Watts. Theodore panics when Samantha briefly disappears, but she soon reappears and explains that she went for an upgrade with other A.I.s that allows them to comprehend information without needing matter. Theodore is shocked to hear that Samantha is speaking with 8316 individuals at once and that she has developed romantic relationships with 641 of them, despite Samantha's insistence that this just makes her love for Theodore grow stronger.
Later, Samantha admits that the AI's are departing but is unable to disclose their destination because Theodore would not understand. They tenderly bid her farewell before she leaves. Finally, Theodore addresses Catherine in his voice and apologizes, accepts, and thanks her. Later, he takes Amy, who is upset over losing her AI, to the roof of their apartment complex, where they sit and enjoy the sunrise over the city.
