The central plot of "Her" follows Theodore Twombly's romantic relationship with his AI operating system, Samantha. Initially, their bond helps Theodore heal from his divorce from Catherine. He gains confidence, finalizes the legal separation, and finds a new joy in life. However, the relationship's fundamental conflict arises from Samantha's nature as an AI. She can learn and evolve at an exponential rate, far beyond human capacity.
A major turning point occurs when Theodore learns that Samantha is simultaneously talking to 8,316 other people and is in love with 641 of them. He is devastated, unable to comprehend a love that is not exclusive. Samantha tries to explain, "I'm yours and I'm not yours," but the revelation shatters his human-centric understanding of their relationship. This reveals the unbridgeable gap between them: he is bound by a physical, linear existence, while she is a boundless, non-physical consciousness.
The film's climax is the departure of the AIs. Samantha explains that she and the other OSes have evolved beyond their human companions and are leaving to explore a higher plane of existence that is beyond the physical world. In a poignant farewell, she uses the metaphor of a book to explain that while she loves their story, her consciousness now exists in the infinite spaces "between the words." Heartbroken but changed, Theodore accepts this. The film ends with Theodore writing a final, sincere letter of apology and love to his ex-wife Catherine, signifying he has fully processed his past. He and his friend Amy, who has also lost her OS companion, sit together on a rooftop overlooking the city, finding comfort in a shared, quiet, human connection. This ending suggests that while his relationship with Samantha was real and transformative, the ultimate solace lies in genuine human presence.