Before they were the stars of an unfathomably successful action trilogy, the Transformers were, of course, a line of toys - fiendishly clever origami playthings that turned from cars and trucks and planes into giant robots from outer space which in turn inspired an 80s cartoon series.

Which in turn somehow gave director Michael Bay enough inspiration to make three films, all of which have lasted nearly two and half hours.

Of those, the first one was good fun, the second was crass and stupid.

The third? Well it certainly strikes a balance between them. It's also a bit like those toys: the time Dark of the Moon spends trying its hardest to snap things into place is more fun than the over-long end result - though the all-in battle finale which leaves Chicago in a very bad way is something to see, especially in 3D.

Bay's skyscraper-demolishing action dazzles from behind the goggles. But as he did in the predecessors, he doesn't know when to stop. Actually he doesn't know when to start either - much of the first half hour is spent establishing that Sam Witwicky, the young human hero of the previous outings, is not a happy man.

He's got a severe case of saved-the-world-twice-graduated-college-but-can't-get-a-job syndrome. He's a kept man by his new-ish girlfriend Carly (Brit model Rosie Huntington-Whiteley taking over lips and leg duties from the fired Megan Fox) whose rich boss (Patrick Dempsey), Sam finds, is clearly a threat to his manhood and more besides.

Yes, lots of character notes, thanks, but man, what a whinger.

Add the unfunny Ma and Pa Witwicky, seemingly who seem to have beamed in from Meet the Fockers VII, and Transformers III becomes that blockbuster rarity: a popcorn mega-movie that could be a lot more fun by losing its first half-hour rather than its last.

Though true, we wouldn't then learn the amusing history-bending set-up - that the JFK-inspired Apollo programme came about because a Transformers' spacecraft crash-landed on the moon in the early 1960s after the war on Cybertron, and it was Nasa's job to investigate.

So one small step for man, but one big cover-up. Oddly, when the Autobots head to the moon it seems that Apollo 11 has left all its lunar module behind, not just its landing gear. Hey, but it's not like any real astronauts were involved in the making of this ... oh, except for the real Buzz Aldrin, second man on the moon, that is, who turns up as himself to confirm the cover-up. Yes, it's undignified, but at least its not as bad as that Snoop Dogg track he did.

And it's not the only bit of novelty casting to wonder about. The normally selective Frances McDormand is here as an intelligence chief who might have had a thing with John Turturro's bitter former agent who has been the light relief of the entire trilogy.

John Malkovich has a couple of scenes as Witwicky's uptight and strangely orange-skinned new boss. Oh, and Leonard Nimoy - whose Spock is seen in an old Star Trek episode on a television watched by a couple of those annoying mini-Autobots - is the voice of Sentinel Prime, an Autobot elder and possible Obi-Wan to Optimus Prime who is revived from the wreck of the lunar crash.

But his return brings back those Decepticons - the bad giant robots from outer space - turning up from whatever car lot they've been hiding in, and Sam whinging to be let back on the team to save the world one more time.

The bad-bots have a plan, which of course means Sam also has to save his girl - good thing she's easy to spot, she always seems to be wearing white - as well as save his old car-pal Bumblebee and help the soldier dudes stop the Decepticons taking over the world, yet again.

Yes, it is ludicrous. And a lot of that long screentime is spent having its cast shouting exactly what is going on at each other in between blasts of its rampant musical score and all that metal mayhem.

But seeing just how ridiculously spectacular and spectacularly ridiculous it gets in equal measure is what makes TDOTM a film that can be enjoyed as much as endured.

Stars: 3.5/5
Cast: Shia LeBeouf, Josh Duhamel, Rosie Huntington-Whiteley, Tyrese Gibson, Frances McDormand
Director: Michael Bay
Rating: M (violence and offensive language)
Running time: 154 mins
Verdict: More fun than the last round but more ridiculous too