The title role in the new film by the wildly inventive American behind Being John Malkovich and Adaptation is played by Scarlett Johansson. Her character's name is Samantha, but she never appears on screen. She is, quite literally, a disembodied voice - Johansson's pert and husky tones were never better deployed - who comes to be the centre of the protagonist's life.
That protagonist is the improbably named Theodore Twombly (Phoenix), a lonesome thirtysomething still pining for his lost marriage to Catherine (Mara).
Theodore was once a journalist of some repute but now composes heartfelt, computer-handwritten letters for paying customers. He is very good at it, too, living proof of George Burns' belief that if you can fake sincerity, you've got it made.
You will infer that the film is set in the future, but it's a near one. In an opening sequence of exemplary economy, Jonze, who wrote the script, evokes a world just beyond reality.
Shooting in Shanghai and Los Angeles and using minimal CGI, he creates a future whose similarity to the present is spooky. And spookier still is the ease with which we accept the central relationship, because Samantha is an operating system.