If I'm honest, the first time I heard they were recasting Kirk and Spock all I could think of was Steve Martin playing Sergeant Bilko. Or Steve Martin playing Inspector Clouseau. Neither of which are likely to go down in cinematic history as anything other than reasons to shake Steve Martin and shout "What were you thinking?"

There are characters, like James Bond, Batman or Sherlock Holmes, who have arguably survived a succession of re-interpretations, but those characters were first shaped on the printed page. They exist independently from their screen portrayals. In contrast, Phil Silvers and Peter Sellers were so instrumental in shaping their respective roles that to re-cast Bilko and Clouseau is to miss the point. It's like re-casting Seinfeld.

It wasn't easy, in the run up to this film, to know on which side of the fence Star Trek lay. William Shatner is famous these days in the same way that David Hasselhoff or Tom Baker are famous: as a cheerful self-parody. William Shatner is definitely not Captain Kirk. But is Captain Kirk William Shatner?

Star Trek doesn't so much answer that question as make it irrelevant. Part-prequel and part-reboot, it both recreates the familiar characters and reinvents them. It's often a remarkable success, and unexpectedly it's the characters and cast who are the greatest part of that success.

We're watching an alternate origin story of the familiar Star Trek characters, sparked by a time travelling villain who has changed history. In this version of events the young Captain Kirk is a brawling rebel without a cause. Like Shatner's Kirk he's a self-confident womanizer with a ready smile, but unlike Shatner's version he's undisciplined and listless, lacking purpose. In those differences lies an immense amount of latitude. Chris Pine exudes an essence of the classic character, but viewed through a different lens. He's James T. Kirk, had events gone a little differently.

Of the Big Three of Kirk, Spock and McCoy, Pine's performance is the least closely modeled on the original, which makes sense in the context of the story since it's Kirk's history that is most altered. Pine wisely makes no attempt to mimic Shatner's unfairly parodied speaking style, but otherwise embodies a recognizable wry confidence. The remaining main cast members appear to have been free to decide how closely they modelled their performances on the original actors. Doctor McCoy is as close as the film gets to a straightforward impersonation, with Karl Urban evoking DeForest Kelley's southern drawl, scowl and posture with delightful accuracy. Simon Pegg's Scotty, in contrast, retains almost none of James Doohan's stubborn pride, presenting a take on the character that amounts to a Scottish accent and a dollop of enthusiasm. The rest of the cast have far less source material to work with. Zoe Saldana is pleasingly smart and forthright as Uhura, even if as the sole female character she is wearyingly obliged to be the subject of both Kirk and Spock's affections. John Cho's Sulu is a fencer and little else, recalling George Takei's memorable turn with a sword in "The Naked Time," and Anton Yelchin's Chekov, like Pegg's Scotty, seems based on little more than a comedy accent. His difficulty pronouncing "V"s is a nod to the "Nuclear Wessels" scene in Star Trek IV: The Voyage Home (1986), arguably Chekov's only memorable moment in the public imagination. Given how sidelined the ensemble was in the 1960s this approach is entirely faithful to the original show but it's a shame that, beyond Uhura, we aren't gifted the small human moments that might have fleshed these characters out.

That leaves Zachary Quinto as Spock, whose task is in many ways the most challenging. He not only has to interpret Leonard Nimoy's very specific mannerisms and make them fresh, but he has to play convincingly opposite Nimoy. Ironically Spock, more than Kirk, is the emotional core of the film. Like Kirk, we track Spock through childhood as he struggles with bullying and alienation, but over the course of the film he also contends with far greater emotional trials, including bereavement on an epic and intimate scale. His more vulnerable take on Spock even allows us to believe in a previously unthinkable romantic relationship with Uhura. It's of course a fallacy that Spock was implacably emotionless in the original series. Nimoy's deceptive performance was always a nuanced mixture of curiosity, bemusement and dry wit. As befits a younger version of the character Quinto's Spock is even more volatile, his emotions closer to the surface, but we've seen enough flashes of strong emotion from Spock over the years (particularly in the first pilot, "The Cage") that Quinto still feels like the same character.

All this could arguably have been achieved by a straightforward prequel. We've never seen on-screen how the Enterprise crew came together, so throwaway dialogue aside who's to say that Kirk wasn't a screw-up in his younger days? But by explicitly making the film a new version of history—an altered timeline—the creators have neatly side-stepped any lingering nit-picks. More importantly they've allowed their characters to live and breathe in their own right, even as they draw on the originals for inspiration.

And live and breathe they do. We follow Kirk and Spock through their childhoods, literally from birth in the case of Kirk (a birth scene for Spock was filmed but dropped). Kirk's father is killed, and he drifts self-destructively until he finds a new father figure in Captain Christopher Pike, played perfectly by Bruce Greenwood. Spock struggles with his own issues with his loving human mother (Winona Ryder) and Vulcan father (Ben Cross), symbolic of his struggle between emotion and logic. Neither fit into their respective worlds. By the time these young characters meet for the first time at Starfleet Academy, we're already heavily invested in their lives and keen to see how their relationship develops.

In fact, the early part of the film is so successful at reinvigorating the characters that when the young Kirk meets an elderly version of Spock, played by Leonard Nimoy, it feels almost like an intrusion. As a longtime fan of Classic Trek I should be the first to rise to my feet and applaud when Nimoy enters the frame, but by this point in the film we've already accepted the possibility of an altered timeline, and we like where it's taking us. We don't care about the whys and wherefores. Remarkable as it is to see this venerable actor reprise a role he first played 44 years ago (and a full 18 years since his last appearance), it's like watching a biopic of a famous singer only to have the actual singer turn up on screen. It drags you out of the reality that the story has created. Worse, Nimoy gets lumbered with a chunk of exposition that brings the movie to a screeching halt and is so stuffed with cod science and coincidence that it actually makes the story less believable. The emotional core of the scene lies with Nimoy not Pine, in a way that only distances the audience. Being charitable, it's easy to see why the idea of placating old Trek fans seemed important to the writers, but it's a baton the film has already passed.

Fortunately Nimoy gets a much better scene at the end of the film in which he and Quinto play off each other. The two Spocks are able to connect across the decades in a way that genuinely benefits from Nimoy's performance as a man more comfortable in his skin. This is the passing of the torch the film craves, but still doesn't truly need.

Kirk's detour to have the plot explained to him unfortunately serves to underline the weakness of that plot. The story of a vengeful working class Romulan, Nero (Eric Bana), is pretty basic stuff that somehow becomes ridiculously convoluted. Nero blames Spock for his planet's destruction for no real reason except that he was late saving the galaxy. Even with Bana's considerable acting chops Nero comes across as a one dimensional thug whose grief is impossible to relate to. Both Nero and Spock then fall through a black hole which just happens to spit Nero out 25 years earlier than Spock, right in front of the starship containing Kirk's father. Nero then hangs around for 25 years waiting for his next scene.

The plotting throughout the film leans heavily on this kind of contrivance. Spock chooses to boot Kirk off the ship rather than lock him in the brig, and Kirk just happens to be marooned on the same planet, in the same spot, as both the older Spock and Montgomery Scott, who just happen to be able to beam back onto the starship travelling at warp speed several light years away. Some of this, such as familiar characters coming together, can be put down to the sense of destiny inherent in a prequel. Some is just lazy plotting. It results in a bitty—dare I say illogical—story that's unified by the characters rather than dramatic inevitability.

The scientific aspects of the plot are equally nonsensical, even for Star Trek. I can forgive the "red matter" that causes black holes—that's the plot MacGuffin and it gets a free pass—but we're told that a single exploding sun threatens the entire galaxy and destroys Romulus within a few weeks without anyone seeing it coming. Delta Vega is an ice world with a lonely Federation outpost which apparently orbits Vulcan so closely that the planet looms in its sky. You can drill a hole to a planet's core with a laser and then simply drop stuff all the way to the centre. The magnetic field of Saturn's Rings is more significant for hiding a starship that the honking big magnetic field of the planet itself. And so on. Maybe a mainstream audience wouldn't notice or care, but these mistakes are annoying because they could have been so easily fixed.

Fortunately it's relatively easy to ignore the flaws. Like last year's Iron Man, what carries the film are the charismatic characters and an infectious energy, much of it successfully recalling the vibrancy of the original television series. Star Trek has been many things to many people over the years, but it has never been as brash and colourful as the original 1960s television series. Viewed today there's much about the original Trek that seems kitsch and of its time, but it has an undeniable charm. It balances its lofty ideals and progressive attitudes with pulp imagery, fisticuffs and bickering. It's rough around the edges, anchored in a tradition of action and adventure. Roberto Orci & Alex Kurtzman's script and J.J. Abrams's direction manage to recapture that vitality, whilst comprehensively modernising the sets, costumes, characters, language, and tone. Even in hindsight it's an unenviable task. Every creative decision must have been a potential minefield: how to take a set of sixties creative decisions about bright primary colours, mini-skirts, and simple sets and update them for a modern sensibility; and how do it without alienating millions of existing fans.

Accordingly, the script goes out of its way to inject the humour of the original series back into Star Trek. Too much humour in places. There are plenty of moments that come across as simply goofy. Amidst the fun of McCoy's repeated hypospray injections of Kirk, his balloon hands are possibly a joke too far. And did Scotty really need a patronizingly-written midget alien sidekick? Or an arcade-game sequence in the Engineering water pipes? Some of this material should really have found its way to the cutting room floor. Nonetheless, for every joke that misses there's one that hits. All the familiar catchphrases are worked into the story ("Live long and prosper," "Dammit Jim . . . ") in ways that feel organic to the moment. This is no mean feat; it's like having Sherlock Holmes say "Elementary, my Dear Watson" and making it feel entirely natural. For long-term fans not content with Nimoy's appearance there are countless subtle tips-of-the-hat to the established history of Star Trek, from the overt (the Kobayashi Maru "no-win scenario") to the obscure (Admiral Komack).

This is a truly cinematic Star Trek film. For the first time in a long time we're treated to a Trek movie that doesn't feel like an extended television episode. The story is epic, the visuals top notch, and the pacing every bit that of a modern blockbuster. There's a sweep to the setting, a scale to the action sequences, and an emotional weight derived from following the main characters from their humble beginnings. The opening sequence in particular, in which Kirk's father heroically fights the good fight as his son is born, is fantastic stuff carried by Michael Giacchino's sweeping score. I'll admit to misting up in a Star Trek movie for the first time since seeing Spock die all those years ago in Star Trek II: The Wrath of Khan (1982).

It's true that there are no big philosophical speeches in this film. Kirk doesn't pontificate on the human condition, nor does he pull off a clever bluff. That's not to say that the story lacks depth, but it's chiefly the depth of a coming of age movie, not the depth of which good science fiction or even good Star Trek is capable. There's a case to be made that if the movie series continues in this vein then this new vision of Trek is a dumbed-down one. That would however be ignoring all the previous Trek movies, which fairly uniformly jettisoned high falutin' ideals for an extra helping of sentiment and a baddie they could kill at the end. And, frankly, most of those weren't nearly this entertaining. The old, philosophical Star Trek may yet live on in a new television series, but if the next film offers nothing more than enjoyable characterization—and a significantly better story—that will suit me just fine.

There's still the question, as there was for Bond after Casino Royale (2006) or Batman after Batman Begins (2005), of how to tell another Star Trek story and hold onto the rawness that the audience liked about this origin story. By the end of the film we've arrived at a situation that closely resembles the original Star Trek series, except that none of those episodes need happen in this timeline. The future of these characters is still unwritten. The innate cleverness of this set-up is the opportunity to tell stories in the original era unfettered by forty years of continuity. The difficult question is how to tell one of those stories in a new way.

That's a question for another day, however. What's more important is that I very much want to see that next story. And I have almost no desire to shake J.J.Abrams and shout "What were you thinking?"

Iain Clark was born in the same year Star Trek was cancelled. He has contributed a number of TV and film reviews to Strange Horizons, and lives in the North of England.