In a near-future mega-city, we meet Theodore Twombly and his acquaintances. They live in a sort of slackers' paradise where, in spite of their trivial occupations, they have lovely casual clothes, great cosmetic dentistry, and inhabit large, luxurious apartments. Joaquin Phoenix is totally believable as Twombly, whose little job is writing love letters for other people. Like the few people he knows face-to-face, he is most concerned about his "personal growth", and relationships are disposable when they no longer serve that selfish goal. He is unsettled by real contact, and we go with him on an incredibly hot blind date which is more than he can stand.
But when Twombly casually installs an artificial intelligence on his computer, he really falls in love with it - and it falls in love with him. Scarlett Johanssen as the AI, with her voice alone, is his lover, friend, and obsession. She learns how to interact most effectively with her human, and part of that turns out to make her even colder and more self-obsessed than the humans. There's a shock in store for Twombly and us.
Jonze conjures up a superficially beautiful way to live, then exposes the loneliness of it. This is a film that sucks you into its world if you will just relax and enjoy the quiet pace of the plot, the convincing acting and the occasional surprise.