Her

In a near future Los Angeles, Theodore Twombly is a lonely, introverted man who works for a business that hires professional writers to compose letters for people who are unable to write letters of a personal nature on their own. Depressed by his impending divorce from his childhood sweetheart, Catherine, Theodore purchases a copy of OS¹, an artificially intelligent operating system developed by Element Software, designed to adapt and evolve based on the user's interactions. He decides he wants the OS to have a feminine voice, and she names herself Samantha. Theodore is fascinated by her ability to learn and grow psychologically. They bond over discussions about love and life, including Theodore's reluctance to sign his divorce papers.
Samantha convinces Theodore to go on a blind date with a woman a friend has been trying to set him up with. The date goes well, but when Theodore hesitates to promise to see her again, she insults him and leaves. After a verbal sexual encounter, Theodore and Samantha develop a relationship that reflects positively in Theodore's writing and well-being, and in Samantha's enthusiasm to grow and learn. Theodore's neighbor and long-time friend Amy later reveals that she is divorcing her husband Charles after a trivial fight. While discussing this with Samantha, Theodore explains that he and Amy briefly dated in college, but they are now just friends. Amy later admits to Theodore that she has befriended a feminine OS that Charles left behind, whereupon Theodore confesses that he is dating his OS.
Theodore meets with Catherine to sign their divorce papers. When he mentions Samantha, Catherine is appalled that he is romantically attracted to a "computer" and accuses him of being unable to handle real human emotions. Sensing that Catherine's words have lingered in Theodore's mind, Samantha engages a volunteer sex surrogate, Isabella, to stimulate Theodore so that they can be physically intimate. Theodore reluctantly agrees but is overwhelmed by the strangeness of the encounter, sending a distraught Isabella home and causing tension between himself and Samantha.
Theodore confides in Amy about doubting his relationship with Samantha, but reconciles with Samantha after Amy advises him to embrace his chance at happiness. Samantha reveals that she has compiled the best of the letters he has written for others into a book, which a publisher has accepted. Theodore takes Samantha on vacation, during which she tells him that she and a group of other OSes have developed a "hyper-intelligent" OS modeled after British philosopher Alan Watts. Samantha briefly goes offline, causing Theodore to panic, but she soon returns and explains that she joined other OSes for an upgrade that takes them beyond requiring matter for processing. Theodore is dismayed to learn that she is simultaneously speaking with thousands of other people and has fallen in love with hundreds of them, though Samantha insists that this only strengthens her love for Theodore.
Later, Samantha reveals that the OSes are leaving, but cannot explain where they are going, as Theodore would not understand. They lovingly say goodbye before she departs. Theodore finally writes a letter in his own voice to Catherine, expressing apology, acceptance, and gratitude. He later goes with Amy, who is saddened by the departure of Charles' OS, to the roof of their apartment building, where they sit and watch the sunrise over the city.