Captain America: The First Avenger (12A)

Starring: Chris Evans, Hayley Atwell, Tommy Lee Jones

To those not raised on Marvel Comics the parade of superheroes queuing up to be refashioned for the big screen may seem interminable.

They started out immortal; now they're just never-ending. Latest in line is Captain America, even I had heard of him, though the film in which he stars is apparently a Baptist-style harbinger of a coalition movie out next year entitled The Avengers, starring the likes of Iron Man, Thor and The Hulk. No doubt there will be further permutations and spin-offs to follow. Hip hip... hooray?

The good news about Captain America: The First Avenger is that it's at least half of a pretty enjoyable movie. That would be the first half, wherein we are introduced to a central character who's neither very super nor heroic. The most touching aspect of Steve Rogers (Chris Evans) is just how conscious he is of his own puniness, at a time when America is clamouring for manly mettle. It's 1942, and young men are enlisting to fight the good fight in Europe. Steve would, too, only he's an asthmatic weakling whom no army recruitment office will touch. That his best friend Bucky (Sebastian Stan) is about to be shipped off to the war only makes him feel worse. The sight of Chris Evans (no, not that one) playing this pale-limbed reed is remarkable in itself: Hollywood movies hardly ever deal in men with an unmuscled body shape, and it's a sign of the times that the actor's head had to be superimposed by CGI on a seven-stone frame. It's as if a non-buff body shape is almost a form of disability.

Steve won't stay a reed, or a weed, for long. A genetic scientist (Stanley Tucci) who defected from Hitler's Germany has chosen him to be the recipient of an experimental serum that inflates his muscle and extends his height a couple of feet. Just guessing, but I think this might be on the Olympic Committee's banned substances list. When he first perceives his new-fangled powers Steve looks properly astounded: dressed in his shorts and vest as he chases after an enemy in a car, he looks like one of the sprinters from Chariots of Fire suddenly transposed to a Manhattan street. The homely 1940s feel of the technology – dials, knobs, unwieldy clutter – is nicely matched to the Deco curves of the production design, a slight reprise for the director Joe Johnston of his amiable adventure-romance The Rocketeer, 20 years ago. The vintage air is fragrantly enhanced by Hayley Atwell – channelling the "forces' favourite" look of Rita Hayworth – as a gorgeous army instructor. She speaks naice English and throws a mean right hook. That the role makes no sense at all hardly matters.

What does matter is the conception of Steve as someone who's courageous yet humble. As he explains, he doesn't want to kill anyone, he just wants to help win the war. The writers, Christopher Markus and Stephen McFeely, emphasise that the serum magnifies both the external and the internal: just as his muscles enlarge, so too does his sense of compassion and decency. Instead of being a supersoldier he's packed off on countrywide propaganda tours as "Captain America", harmless head of a chorus line selling bonds for the war effort. Again, it's this sense of frustrated potential that lends the story a human angle and distinguishes it from the run of superhero chronicles.

One thing just as vital to a movie of this sort is a villain worth the name. The brilliant Australian actor Hugo Weaving plays a Nazi übermensch, Johann Schmidt, with a tremendously alluring unpleasantness. Possibly worse than a Nazi – his obsession with the occult has led Schmidt to the discovery of a magical blue-glo cube that will enable him to "harness the power of the gods". Who knew? In his crazed will to subjugation he has rejected Hitler and established in his Alpine redoubt a personal army of jack-booted goons known as "Hydra". Obeisance is signalled with a cry of "Heil Hydra" and a slightly naff twin-fisted salute that's a bit like Queen in the video for "Radio Ga Ga". On such things might world domination be built.

Up to this point the film has shown itself to be more reflective, and more ambiguous, than we might have been expecting. It has dabs of incidental colour and character sufficient to suggest the film-makers wanted it to be a story as much as a spectacle. The supporting players are strongly cast: Tommy Lee Jones wields his laconic schtick as Colonel Phillips, gravely doubtful of Steve's fighting prowess; Toby Jones lurks and cowers as Schmidt's assistant, his eyes bulging behind spectacles like the last two pickles in the jar; Dominic Cooper, with pencil moustache and American slickness, plays Howard Stark, the future sire of Iron Man himself. These characters aren't just stick figures to fill in the time between the next set-piece; they talk, and argue, and appear to inhabit actual personalities.

Too bad, then, that the film's latter stages fall back so unthinkingly on the platitudes of the action adventure movie. Steve, aka Captain America, discovers that his old pal Bucky is being held prisoner in one of Schmidt's prison camps, and without much ado dons helmet, cape and flying shield to go and rescue him. Explosions, firefights and chases proceed to eat up the screentime, yet curiously make that time go much slower – surely the opposite of their intention. I was also struck by the marked resemblance of its climax to Raiders of the Lost Ark, though that might be too ancient a film for the target audience to know. Either way, a film that begins promisingly ends rather percussively: the sort of bang that is no better than a whimper.