When the playground scuffle over whose turn it was to play Captain America started, you can picture the young Joss Whedon arriving with a homemade shield. A Marvel fan since childhood, the writer-director's confident take on the Avengers series carries the mark of a piece of expensively assembled fan fiction, albeit stitched together with a lightness and wit that will just about carry the casual viewer through two-and-a-half-hours of comic geek nirvana.

That said, those unfamiliar with the Avengers universe should ride out the opening act with a glossary to hand. The nefarious demigod Loki (Tom Hiddleston) has teamed up with alien race the Chitauri and stolen the Tesseract – a powerful energy source that could potentially destroy the world. In response Nick Fury (Samuel L Jackson), director of shady government agency SHIELD calls on Iron Man, Thor, the Incredible Hulk, Black Widow, Hawkeye and Captain America to whack the god of mischief into submission.

Drawing together the meaty backstories of a half-dozen iconic heroes is no easy task – even with the help of the blockbuster Iron Man, Thor and Captain America movies that set up this showcase – but Whedon proves himself a master plate-spinner. Each hero is given a convincing story arc, even if – as in the case of Jeremy Renner's Hawkeye – it's as shallow as revoking his lone wolf status to allow him to team up. Crucially, the wise-cracking id of Robert Downey Jnr's Iron Man is balanced out by instilling a hearty dose of fear in one of the team's central figures, thanks to a beautifully honed performance by Mark Ruffalo. He plays Bruce Banner – the brilliant scientist who turns into the uncontrollable colossus Hulk when angered – as a man afraid of himself. Banner, dressed in clothes that hang off him (as if preparing for the worst) is an exhausted presence, a man so tortured by his divided-self condition that he once put a bullet in his mouth, but "the other guy spat it out".

Unfortunately, this being a comic-book movie, the need for biff-bang-pow tends to prevail. The hero-against-hero slugfest that the comics have been trading on for decades is entertaining at first, but the various combinations of Hulk v Thor v Iron Man quickly blow themselves out. Perhaps it would have helped if they had a properly combative adversary. Hiddleston's Loki, while a gloriously over-boiled caricature of the emotionally crippled boy-man we met in Thor, is backed by a horde of faceless, disposable allies, and it's hard to see how they put up much of a threat.

So the biggest challenge The Avengers face is each other. The greatest conflict is between themselves. There's not much to see after they've sorted out the pecking order, but until then it's fun to watch Whedon pitch his heroes against each other. Child's play, maybe, but entertaining all the same.