Terminator Salvation isn’t the disaster some are making it out to be. Some well-crafted action sequences and the hollow, booming robot sounds that rattle the cinema are worth at least a buck or two of ticket price.

(Admittedly, I’ve been a fan of McG since the first Charlie’s Angels, which combined the shimmering, high-energy fun of an old Hollywood musical with a sword-fighting Crispin Glover.)

Others have written in detail about what went wrong with the movie: how John Connor’s scenes were reportedly added piecemeal to the screenplay, explaining why they feel so redundant and barely half-stitched into the fabric of the film. HowTerminator Salvation fails most spectacularly, though, is as a satisfying sequel.

A good sequel is a complicated tightrope-walk between repetition and variation, and back in 1991 Terminator 2 struck the ideal balance. It took familiar elements and twisted them, so we saw Sarah Connor transform from lonely waitress to pre-apocalyptic warrior; saw the unstoppable high-tech terror of the first film become the desperate underdog when pitted against liquid metal; and saw the superb bad-Terminator-becomes-good-Terminator fake-out (which, yeah, everyone knew in advance, but still).

Now John Connor’s been told he’ll be the Saviour of the World from long before he did anything to justify it, and pop-psychology dictates that’d screw you up into a fascinating wad of dramatic issues. He should be inspiring fascinating madman-or-messiah reactions everywhere he goes. In Terminator Salvation, it’s like the screenwriters were told not to mention the earlier films in any detail – with no time travel talk, especially. John Connor knows that he has to save the young Kyle Reese in order to later send him back in time to become John’s father. We know it too. But for some reason it can’t be said out loud, leaving John shouting about how “the future’s at stake!” without anyone reacting beyond a kind of “Oh, that John…”.

Instead, Terminator Salvation trades on the surface affectations of its sequel status, like Linda Hamilton returning to record some pointless Sarah Connor voiceover tapes, or the much-discussed digital Schwarzenegger cameo, or John Conner saying “I’ll be back” – with an appropriate didja-catch-that-huh? music cue.

Sure, James Cameron used all these tricks too, but he balanced it with masterful high-concept storytelling techniques. His dialogue might occasionally exhibit a blistering case of the George Lucases, but in Terminator 2 his sequel-logic is note-perfect.

One last thing: there’s another pitfall inherent in going back to the same stories again and again, and it’s the reverse of the old ‘show don’t tell’ that’s inevitably bandied around during the first week of any creative writing class. The problem is that some things are much more powerful when they’re just imagined than when they’re actually splashed up on the screen for all to see.

Is the looming apocalyptic robot-war more interesting as a terrifying hypothetical? Reese explaining the future is much more harrowing than any of the flashbacks to the future in the first Terminator. (Okay, okay, except for the iconic scenes of a tank-tread rolling over human skulls, which haunted my misspent, violent-movie-watching youth.)

The first film’s final moments of an approaching storm hold much more menace that the grim reality of Salvation: