Who'd have thought a radioactive spider-bite could prove so versatile? In Spider-Man (2001) nerdy Peter Parker's super-powers were treated as a metaphor for puberty in a quirky tale of boy-meets-girl. In Spider-Man 2 (2004), his powers suffered an embarrassing bout of impotence in the face of crippling self-doubt, although he did win the girl. Now in Spider-Man 3 his powers become as self-destructive as his testosterone-fuelled behaviour, and boy loses girl once more.

This third film in the series is much the same as its predecessors: a small tale of emotional turmoil writ large through vivid metaphor. But where the first two films mostly pulled off a sure-footed balancing act, Spider-Man 3 repeatedly trips over its own feet. The film tries to pack in more characters and plot threads than both of its predecessors put together, and it becomes increasingly clear that the writers have bitten off more than they can chew.

But let's begin at the beginning. Spider-Man 3 eschews the trend of many recent blockbusters by placing its credits at the start of the film, taking the opportunity to incorporate a well-designed recap of the first two films. This is considerate, since the storyline picks up right where the last film left off and freely references characters and events from the previous instalments.

The main storyline follows the romantic ups and downs of Mary Jane Watson (Kirsten Dunst) and Peter Parker (Tobey Maguire). Between Mary Jane's upcoming Broadway musical and Spidey being the flavour of New York, the two are blissfully happy. Naturally it can't last, and the couple gradually become so self-absorbed that they end up pushing one another away.

The film's trio of villains enhance and intrude upon this simple human drama, weaving in and out of Peter and Mary Jane's story in ways so convoluted that any attempt at a brief précis is doomed to failure. Suffice it to say that at the start of the film, Harry Osborn (James Franco) knows Parker's secret and mistakenly wants revenge on Spider-Man for the death of his father in the first movie. We're introduced to Flint Marko (Thomas Haden Church), a small-time hood with a heart of gold who, in an unlikely twist, was the real killer of Peter's Uncle Ben in the first film. It's not long before Marko is accidentally transformed into a being of living sand. The final antagonist is Eddie Brock, played energetically by Topher Grace, a free-wheeling reporter who acts as Peter's mirror image: both are hoping to propose to their long-time girlfriends, and both are looking to win a staff position on the Bugle by photographing Spider-Man.

The movie's main theme is redemption from bad choices. The villains of the piece—known to comics readers as the Green Goblin, the Sandman, and Venom—are significantly adjusted from their comics counterparts in ways which embody this theme. The Sandman, in particular, is a distinct improvement on the two-dimensional original. Osborn and Marko are basically good men who do bad things, in Marko's case because he needs the money to care for his terminally ill daughter. Where Osborn has succumbed to bitterness and blames Spider-Man for his father's mistakes, Marko abdicates responsibility for his own actions: he's just "unlucky." (Although given what happens to him in this film it's hard to argue.) Lastly, Brock—who ultimately will bond with an alien symbiote and become Venom—is an amoral chancer who explicitly makes all the bad choices that Parker has so far avoided.

What really sets Spider-Man 3 apart from its predecessors is that this time Peter Parker's mistakes mirror those of the villains, leading him into a much darker place. Wrapped up in himself, he ends up pushing Mary Jane away, starting down a spiral of arrogance and resentment. It's an interesting direction to take the character, and one which promises some genuine drama. Unfortunately for the audience, the film isn't done throwing villains into the mix.

The alien symbiote which is destined for Eddie Brock first makes a beeline for our beleaguered hero. Bonding with him, this black goo forms a black version of Spider-Man's costume (famous from a comics storyline in the 1980s), granting him increased strength and agility but exaggerating his aggression. In principle this is a neat way to marry the symbiote to Peter's emotional state; in practice it's also an unnecessary complication which skews Peter's fall from grace into caricature. As he slicks his hair down (in a bemusing style presumably intended to denote "evil") and struts a comedy Travolta impersonation, it's increasingly unclear whether he's supposed to be a callous ladies' man or a self-deluding jerk. Perhaps the point is that even under alien influence Peter is basically a nerd, but the film never makes up its mind. There's no rhythm to these scenes, Spider-Man's persona floundering somewhere between Angelus, from TV's Angel, and Mr Bean.

Ironically the symbiote ends up hijacking the very storyline it's supposed to explore. When Peter finally does abruptly come to his senses, the moment is reduced to the superficial discarding of the alien creature. It's a huge anticlimax. This should be the heart of the film—the cathartic moment when Peter realises what he's lost and wonders how he can make amends—but the film deals with it entirely in terms of external symbols, such as the removal of the black costume, rather than by genuinely tackling Peter's inner turmoil. The film has too many demands on its time to linger, so there's no great agonising: he simply decides to stop. As a result this pivotal sequence is reduced to a mere interlude, reminiscent of Superman coming under the influence of Red Kryptonite in Smallville or Superman 3, and with as little emotional significance.

Many of the film's other key moments feel equally unearned, because the characters are always subservient to the plot. Flint Marko is a sympathetic character, played with a soulful dignity by Thomas Haden Church, who is criminally sidelined as a roaring special effect for the bulk of the film. Harry Osborn, whose long-running saga has its roots in the first film, goes from good guy to bad guy and back again so often, and so conveniently, that his growth is fatally undermined. So cluttered is the plot that his decision to use his father's super-serum is almost thrown away in passing, while his repeated character reversals dovetail so neatly with Peter and Mary Jane's storyline as to beggar belief. There's a moment—we'll call it Butler Ex Machina—when a minor manservant suddenly has to reveal a vital piece of exposition because without it most of the film's conclusion, and a sizeable chunk of Harry's characterisation, will come crashing down around its ears.

It's far from the most blatant contrivance in a plot already creaking under the strain of its own implausibility: Flint Marko is ret-conned as Uncle Ben's killer and almost immediately stumbles into a high-energy physics experiment conducted in the open air behind a small wire fence. The never-explained alien parasite just happens to land not twenty feet from where Peter and Mary Jane are sitting; Eddie Brock just happens by the same church as Peter at the same time; Osborn's amnesia disappears as soon as it becomes inconvenient; and the murderous Osborn is suddenly satisfied with sadistic emotional games. The characters feel more like playthings of the gods than real people engaged in meaningful choices. In a world of fantastic events, internal consistency and watertight storytelling are all the more crucial if we're to believe in the characters. While it may be asking too much for a superhero blockbuster to deliver more than resounding action and sound-bite philosophy, this film is trying to be more than just a tale of super-powered fisticuffs and needs to be judged accordingly. The first two films certainly had their fair share of non sequiturs, particularly in the origins of their respective villains, but with fewer characters competing for attention their central narratives flowed far more convincingly.

It's to the credit of the script that it wrings any poignancy from this lumpen mass of contrivance, but somehow it does. This is a hard film to entirely dislike, sharing with its predecessors an underlying sense of sincerity that belies its awkwardness and commercial trappings. The raw materials that make up the film are actually very strong. The characters and performances are generally good, and their shifting interactions are full of potential. It's impossible for the script to miss such ready targets, and throughout the film there are some strong moments plucked from the greedy clutches of the plot. Flint Marko's rebirth as the Sandman is a standout set piece. The pathos of his performance shines strongly through the CGI, giving the lie to the notion that emotion has to stop when the effects begin.

Another of the film's highlights is the scene in which Peter's dinner with Mary Jane disintegrates, aided by an effortlessly charismatic performance from Bruce Campbell in a Monty Python French accent. Indeed, Tobey Maguire is as good in this sequel as he's ever been in the role, aided by a script that lets the character be less of a bystander in his own life. The films have often emphasised Parker's high-school nerd persona to the exclusion of his other characteristics, but in the comics Peter has long outgrown the teenage drippiness he displays on the big screen. His decisiveness here is a welcome breath of fresh air.

There's also a remarkable sense of continuity with the previous films. The same actors reappear in even the most minor roles and there's a real sense that the universe of the films has its own life. In some cases, such as J. K. Simmons's perfect portrayal of J. Jonah Jameson, it's only a shame that they don't steal more of the limelight. Even Harry Osborn's botched storyline benefits immeasurably from its gradual build-up during the previous films.

However, the cumulative effect of the film's strengths is never quite enough to balance out its weaknesses. There are many more flaws than those I've mentioned, including the now obligatory post-9/11 scenes of New Yorkers cheering on their hero, some truly awful news commentators, and a shockingly silly cameo from Spider-Man creator Stan Lee. Although the film's co-writer and director Sam Raimi displays the same firm grasp of storytelling and pulp flourishes as in the previous films, the pacing of the film is wildly uneven, and several action sequences disappear into an impressionistic blur of hyperactive camerawork.

Spider-Man 3 is, by all accounts, the most expensive film ever made. It will also, in all likelihood, be one of the most profitable films ever made. On its opening day of US release it earned the largest one-day box office takings in history, a record it's likely to hold for precisely as long as it takes for the next big blockbuster to be released. Given this kind of absurd Hollywood excess it's perhaps a miracle that Spider-Man 3 tells such a human story, let alone one that focuses as much on character as on spectacle. In this respect the film is almost as remarkable a balancing act as its two predecessors. Unfortunately, for the first time that Hollywood "bigger is better" approach applies on a creative as well as a commercial level, and that's this sequel's critical flaw: it doesn't know when to stop. With clear-sighted, even ruthless, editing this could have been a superior example of its type. But in trying to fuse enough story and characters for two full movies into a single story, the script is forced to artlessly manhandle its characters instead of allowing them to respond naturally. In any film which prides itself on characterisation—even a Hollywood blockbuster—this is a cardinal sin.

So it's a miracle, certainly, that the film is as good as it is, but this time around that's not quite enough. The first two films in this series had led us to expect a little bit more from our miracles.

Iain Clark lives in the North of England with his wife and two cats, who feed him most of his best ideas.