In the high-tech Los Angeles of the near future, Theodore (Joaquin Phoenix) earns a living by making people happy, by composing touching, bespoke personal letters.

But Theodore himself lives alone and lonely, unable to move forward after a broken relationship.

A technology "early adopter", one day he decides to test-run a new personal organiser, whose operating system is an artificial intelligence that listens, understands and empathises with its owner. Sam, as she's known, is secretary, proof reader, companion and confidante; she is super-smart, funny and has the sexy whisky voice of Scarlett Johansson. Theodore falls in love.

Actually, it's mutual. Her is a love story between a man and a device the size of a mobile phone. As with Spike Jonze's previous films, Being John Malkovich and Adaptation, much of the pleasure derives from a preposterous situation treated as being near-normal by its protagonists, and which we, then, also begin to accept.

The love affair between Theodore and Sam plays like any affair between people - with courtship, flirting, passion, growing intimacy, eventually jealousy and mistrust.

"I'm not in a place to commit right now," says he at one point, reflecting the reasons he's alone in the first place. "It doesn't change the way I feel about you," says she, when revealing an interesting aspect of her growing capabilities. And so, as well as being very strange, this is a lovely, clever, complex film about everyday things that govern relationships - commitment, constancy, openness, give and take.

After his brutal, scary performance last year in The Master, Phoenix breaks your heart as the sweet Theodore, while Johansson does wonders with her voice-only assignment. Rooney Mara, Olivia Wilde and Amy Adams are the flesh-and-blood women who, respectively, have been, would like to be, and ought to be part of Theodore's life.