It is hard to believe the Terminator franchise is a quarter of a century old. The first film was an exciting, low-budget action thriller that, like so much science fiction, extrapolated from current trends to predict a dark future for mankind. It launched the careers of Arnold Schwarzenegger and director James Cameron.

The 1991 sequel, Terminator 2 (which was also directed by Cameron and starred Schwarzenegger and Linda Hamilton), was one of the most expensive movies of its era. It boasted revolutionary computer-generated effects and was significant for the way it epitomised a new sci-fi fashion for tough, gun-toting heroines. Like the first film it expressed an unease — if not in society as a whole, then certainly in Hollywood — about computing technology.

Given the spectacular advances of that technology since then, it is not surprising that the theme of the franchise — that our technology threatens our lives and our humanity — retains its power. Of course, the irony of the films is that they themselves are more dependent on computers than any other movie genre. And all too often, the more high-tech such movies become, the less interesting they are as stories.

There was strong evidence of this tendency in the rather tired Terminator 3: Rise of the Machines, the last in the series to star Schwarzenegger. The latest and fourth instalment is an undeniable improvement — fresher, more inventive and much more exciting. Directed by an action wizard who goes by the name of McG (real name Joseph McGinty Nichol), it boasts terrific chases and battle sequences.

Nevertheless, its technical and visual virtuosity is matched by a video-game soullessness.

Terminator: Salvation is intended to be the first of a new trilogy. Written by John Brancato and Michael Ferris (who scripted T3), it breaks with its predecessors by abandoning the standard plot concerning the efforts of a humanoid robot sent back in time to murder a particular human being. It is also set almost entirely in a post-nuclear future, not long after Skynet, the self-aware military computer system which manufactures Terminators, has launched a massive attack against its human makers. Thematically it has more in common with 1982’s Blade Runner — and its concern with what makes us human — than its three predecessors, though it is not as thoughtful.

John Connor (Christian Bale), having survived assassination attempts in the previous films, has become a leader of the human resistance. As the action begins, he and his superiors are planning an attack on Skynet. The human soldiers dress like people who have seen many post-apocalyptic films, favouring a lot of distressed leather. Even the Terminators seem to have adopted a grunge aesthetic, some of them wearing what look like leather headbands around their chrome skulls. In a raid on a Skynet facility, Connor discovers that the machine enemy is performing some kind of experiment on human prisoners. Soon afterwards a man named Marcus (Sam Worthington) turns up whose last memory was being executed in a Texas prison for murder. Strong and skilled at combat, he rescues one of Connor’s downed pilots, an exotic looking young woman played by the wonderfully named Moon Bloodgood.

Marcus turns out to be part machine — a new prototype of Terminator — though he believes he is a human being. Connor decides to have him disassembled. However, as if realising that Marcus may hold the key to humanity’s survival, the pilot helps him escape.

Thanks to powerful screen presence, Worthington is likely to join Eric Bana and Hugh Jackman in the ranks of Australians conquering Hollywood. Raspy-voiced Bale, as so often, is one-note angry — he may be one of the more overrated actors of his generation.

McG was a famous music video director and subtlety is not his strong point. Nor is consistency. Much of the film is shot in muted bluish hues, as if the light has been leached out of the post-nuclear future; in other scenes someone seems to have forgotten to attach the filter to the camera.

And while many of the effects are dazzling, and the stunt camerawork a triumph of kinetic filmmaking, there are sequences in which the levels of craft and inventiveness fall away. These include a scene in a scary Terminator HQ where flame buckets of the kind used in heavy metal concerts belch balls of fire for no apparent reason. Nor does it make sense that shiny metal Terminator robots would make growling noises like the sharks in the awful sequels to Jaws.

Terminator Salvation is appropriately dedicated to the late Stan Winston, the special-effects genius who invented the original Terminator and many of the celebrated effects in films like Jurassic Park and Predator.