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.
Specifically, she is OS1, a far-from-outrageous extrapolation of Apple's Siri. Billed as "the first artificially intelligent operating system" and bought, like an iTunes card, from mall booths, OS1 lets the user assign a gender and voice, but - as Theodore will discover - how it grows up is beyond user control.
When Samantha asks Theodore, "Are my feelings real or just programming?", we may have some idea what's coming, but Theodore himself seems blind to it.
To say Samantha is both useful and efficient is an understatement: within minutes, she has cleaned up his hard drive and email folders. Later, she sets him up with a date (it's a magnificent disaster) and ... well, the rest is spoilers.
The carefully composed production design, which eliminates blue from the palette, creates a world of visual oddity that helps inveigle us into its improbable central proposition: video games are holographic and computers have no keyboards, but all the men wear geeky, waist-high beltless trousers and Theodore's unimaginably smart phone, called a book, looks like a leather-trimmed cigarette case of the 50s.
The subtext of all this - that technology has not so much interfered with human interaction as replaced it - is none too original and it would be overstating the case to suggest that this piece of post-digital whimsy makes any sort of profound statement.
But it is devoid of the knowing tricksiness that I, for one, found so irritating about Malkovich and Adaptation. It's driven by a sensibility that is romantic in all the senses of that word. And that can't be a bad thing.
Cast: Joaquin Phoenix, Amy Adams, Rooney Mara, Olivia Wilde, Scarlett Johansson
Director: Spike Jonze
Running time: 125 mins
Rating: R16 (sexual content, language)
Verdict: Spooky love story