Many of us are infatuated with our computers. They connect us to all kinds of pleasures. But what would happen if we really did fall in love with our operating systems? This is the premise of Her, Spike Jonze’s postmodern romantic comedy.

In a near-future Los Angeles, Theodore Twombly (Jaoquin Phoenix) writes love letters for clients of the Beautiful Handwritten Letters company. ‘Sad and mopey’ after breaking up with his wife, it seems Theo is in need of one of his own romantic pick-me-ups.

After seeing an advert for the OS1 (‘not just an operating system, it’s a consciousness’) Theo is quick to buy one. As voiced by Scarlett Johansson ‘Samantha’ is endowed with human qualities such as intuition, a sense of humour and the ability to learn and grow through experience. The OS1 quickly takes over Theo’s life. ‘She’ organises the hard drive of his computer, selects the best of his letters and puts them in a publishable collection, watches him sleep. But most of all she talks to him through his mobile/iPod device.

Before too long they are intimate. ‘I feel like I can say anything to you,’ says Theo. Soon they are having virtual sex together and achieving simultaneous orgasms. Like a proper couple, they goof around, interrupt each other’s speech and go to the beach and mountains. They also argue, especially after Samantha sets up Theo with a surrogate partner to provide a body for her, an arrangement that makes him uncomfortable.

As time goes on, with Samantha unsettled by the speed of her evolving feelings, we begin to doubt whether the film will have a happy ending.

There is plenty to enjoy in Her. The acting is excellent. As Theo, Oscar-nominated Joaquin Pheonix is amiable, sensitive and warm-hearted; he looks like Tom Selleck channelling Elijah Wood’s big-eyed soulfulness. Scarlett Johansson, even in her disembodied state, is as sexy as Jessica Rabbit (from Who Killed Roger Rabbit?) and makes the idea of a man falling in love with his computer entirely convincing.

Comedy highlights include Theo’s bizarre sex-line phone chat with SexyKitten (Kristen Wiig), who eventually demands a virtual dead cat. ‘I’m wrapping its tail around your neck now,’ says Theo, taking the kinkiness in his bemused stride. His favourite hologram video game features a hilariously foul-mouthed marshmallow-headed alien (voiced by Spike Jonze himself). Elsewhere, best friend Amy’s video game Perfect Mom features Mom giving the kids breakfast and doing the school run (You’re class, Mom! 50 pts); when she fails, the kids trash the kitchen. In both games, the graphics are incongruously clunky for the film’s sophisticated future.

Writer/Director Jonze clearly had fun creating this not-too-distant utopia. His Los Angeles is full of tasteful skyscrapers (one with retro gable ends) and orderliness. In public people drift around calmly in their own techno bubbles, earpieces as companions. Denim, it seems, has been outlawed; everyone wears fawn slacks with high waistbands.

Her is a very talky film. Ideas about what is ‘real’ are debated by Samantha and Theo. Wise-sounding phrases are sprinkled around like confetti: ‘the past is just a story we tell ourselves;’ ‘we’re all made of matter;’ ‘we’re all under the same blanket.’ But they go in one ear and out the next – we do not leave the cinema feeling particularly enlightened.

If Jonze had pruned 30 minutes from its running time and injected a bit more humour, Her could have been a top-class satirical romcom, rather than a very entertaining one. As it stands, despite faultless performances and some lovely imaginative flourishes, this film about sex and relationships doesn’t quite hit the G-spot.