He's already made one 'trilogy of a lifetime' - and now New Zealand director Peter Jackson is back in pre-industrial Middle-earth with the irresistible start of a second.

An Unexpected Journey is his fourth, three-hour movie based on the imagination of a boy from the Wake Green Road who used to play outdoors in Birmingham's Moseley Bog.

That, in itself, is an incredible journey, neatly timed to coincide with the 75th anniversary of the novel first being published.

And here's the great news: this is not The Phantom Menace in disguise, the stodgy film which wrecked the Star Wars' prequel trilogy.

In fact, with another two blockbuster-sized adventures still to come, there can only be one verdict about The Hobbit: It's tree-mendous.

Against the odds, and despite plenty of flaws and unnecessary extra padding sourced from Tolkien's own notes for The Lord of the Rings, this new forerunner to JRR Tolkien's The Lord of the Rings is set to have merit on its own terms.

The breathtaking action sequences make it the most exciting film of 2012.

And, as well as being a lot of fun, it's a technological groundbreaker, too.

The story is about a comfortable hobbit called Bilbo Baggins (Martin Freeman), setting out on a journey he doesn't want to make - and becoming a man en route.

Arriving at his door is Sir Ian McKellen, returning for Jackson as the ancient, but persuasively-wise wizard, Gandalf the Grey.

"Courage is all about knowing not when to take a life, but to spare one," he says.

This time Gandalf has 13 bearded friends in tow.

The dwarves (Tolkien's spelling) no longer have a kingdom of their own but are a 'once mighty people brought low'.

They raid Baggins' pantry during an unexpected visit, rather like 21st century gatecrashers in fancy dress would go looking for booze after spotting a misplaced social networking alert.

Bilbo is shocked, but his own understanding of the importance of having a place called home will later help him to protect the dwarves' journey into the unknown together.

One that will encompass everything from Elves to Goblins, Orcs and deadly Wargs, as well as the sinister figure of the Necromancer (Benedict Cumberbatch).

Since the dragon Smaug hasn't been seen for 60 years, it seems like an unmissable opportunity for this motley crew led by the legendary warrior Thorin Oakenshield (Spooks' Richard Armitage) to seize back their homeland.

Especially as any smell of their secret hobbit weapon Bilbo Baggins is 'all but unknown' to the creature.

Many Tolkien readers will regret the fact that what was a bright and breezy 300-page novel should have become a silver screen trilogy, with the first part alone lasting 169 minutes. But it would be churlish to complain too much.

With legal and financial difficulties plaguing the project for years, and planned director Guillermo Del Toro (Pan's Labyrinth) dropping out in May, 2010, this particular 'Hobbit' was going nowhere.

It has been saved by Warner Bros' need for another franchise after the Harry Potter series.

The Hobbit is an old-fashioned Raiders of the Lost Ark-style adventure film. It's also a technical experiment, too, with Jackson following Tolkien's own pioneering literary spirit by bravely releasing it on some screens in a high definition 3D format playing at 48 frames per second - twice the usual rate.

The expectation is that viewers will think 'high frame rate' technology offers a smoother ride in 3D.

Agreed. But in terms of the overall look of the film, I found the high definition aspect to be hit and miss in The Unexpected Journey even though I was at the first UK cinema screening of The Hobbit at the Odeon Leicester Square. When it begins, its high definition look is so unfamiliar you'll be wondering what you are watching - especially as lots of scene-setting voiceover makes it difficult to listen intently and to study the visuals at the same time.

Eventually, you get used to The Hobbit's curious sense of pace, place and time.

And even begin to appreciate what the technology has done in terms of make-up and in recording such free-flowing patterns of movement on a spectacular, geological scale.

As well as a real sense of discovery, endeavour, camaraderie and bravery, angry giants can reshape the landscape single handedly and one plunge in particular reaches a Toy Story 3 level of excitement.

In contrast, the returning Cate Blanchett looks radiant by the light of a crescent moon.

But since her character Galadriel isn't in the book, she naturally doesn't get much to do in scenes which also feature the return of Christopher Lee's Saruman (shot in four days in England because of his age).

Elsewhere, the sense of movement is quicker than it was in Avatar and the flight of birds, in particular, more accurately reproduced.

One stunning flying sequence might be termed 'the talons of terror and the feathers of freedom' for its ability to transfer the film's ability to make actors look small indoors into the wider animal kingdom.

An Unexpected Journey opens 60 years before the rest of the film takes place, with Sir Ian Holm's Elder Bilbo telling fellow returnee Elijah Wood: "My dear (nephew) Frodo, I have told you the truth. I may not have told you all of it...

"I am old now, I am not the same hobbit I once was. Time for you to know what really happened."

While The Unexpected Journey lacks the true sense of Fellowship that Jackson found in his first Lord of the Rings film, such are the challenges in store here that viewers will develop more genuine emotional connections.

It's great to see Andy Serkis returning as that 'small slimy creature' Gollum, this time challenging Bilbo to solve a series of riddles - only to lose his most prized possession, a simple gold ring that's tied to the fate of all Middle-earth in ways that Bilbo certainly cannot know.

These scenes offer some genuine, Skyfall-quality acting respite from what is otherwise such an impressively-sustained action movie.

Seven of the dwarves are played by New Zealand actors and it's a shame that they didn't have one more.

The accent of Irish star James Nesbitt (Bofur) stands out so much that despite his almost impenetrable disguise, he still ends up looking Little Britainesque, rather like David Walliams in Great Expectations.

Other big names, from Ken Stott as the dwarf Balin ('We're wider, not shorter') to Barry Humphries' Great Goblin are much more in character. The excellent cinematography is by Jackson's regular Oscar-winning partner Andrew Lesnie. We'll never know if Pan's Labyrinth director Guillermo Del Toro - one of the script's four co-writers along with the returning Fran Walsh, Philippa Boyens and Jackson himself - would have done a better job.

Peter Jackson's return is bold enough, and The Hobbit is generally good enough, for us not to even ask the question.