The past three Terminator films - James Cameron's masterpieces The Terminator and Terminator 2: Judgment Day, and Jonathan Mostow's solid, belated entry Terminator 3: Rise Of The Machines - may have been dominated by the big, steely, larger than life presence of Arnold Schwarzenegger, but they were actually about something else entirely. The focal point of the Terminator series has always been the relationship between Sarah Connor (Linda Hamilton) and her son, John Connor.

In The Terminator, Sarah was protecting her not-yet-conceived son from Arnold Schwarzenegger's indestructible cyborg sent back from the future. In the second, John was a young teenager (Edward Furlong) who this time bonded with a reprogrammed model of Schwarzenegger's indestructible cyborg, whose role was now as protector. In the third film, John Connor is a young adult, attempting to halt the course of history, which will see a computer-led destruction of humankind. Though filled with action, and driven by apocalyptic set-ups and strong themes about man's use of technology, the Terminator films really rested on the shoulders of Sarah and John Connor. The makers of this "reboot" of the series, however, seem to have missed this vital piece of the Terminator puzzle.

Though John Connor (played with characteristic intensity by Christian Bale) is very much a part of this story - which is set after Skynet (the computer system which has become self-aware) has instigated a nuclear holocaust and set about destroying humankind, ushering in its first wave of "primitive" terminators - he becomes increasingly sidelined by the character of Marcus Wright (a rather flat Sam Worthington), a former Death Row prisoner who may hold the key to Skynet's creation of new technology which could spell the end for humankind.

Though the action is well staged by director McG (who also maintains a consistently gritty visual aesthetic), and there are entertaining nods (some admittedly extremely tacky, particularly the use of The Gunners' signature tune "You Could Be Mine") to the past films, the plotline of Terminator Salvation ill-advisedly veers away from what has grounded the films in the past. Every time the camera stays with the dull and insufficiently developed Marcus Wright, you just want to know what's happening with John Connor, and his efforts to rally the human resistance. This is where all the other films have been inexorably leading to, and Marcus Wright just gets in the way of this classic sci-fi journey.

It certainly doesn't make Terminator Salvation a bad film, but it stops it from being the great one that it could have been. It's exciting and propulsive, but without the required core of emotion. That might not have seemed too important to the Terminator films, but once it's gone, you really notice.