A sensitive and soulful man earns a living by writing personal letters for other people. Left heartbroken after his marriage ends, Theodore (Joaquin Phoenix) becomes fascinated with a new operating system which reportedly develops into an intuitive and unique entity in its own right. He starts the program and meets "Samantha" (Scarlett Johansson), whose bright voice reveals a sensitive, playful personality. Though "friends" initially, the relationship soon deepens into love.