The surprise thing about A Sound of Thunder, the sci-fi thriller based on the Ray Bradbury short story about time travel, is the quality of acting. Ed Burns, Catherine McCormack and Ben Kingsley aren't aiming for Oscars, or even accolades, with their performances, but their choices make this over-budgeted B-movie fun to watch.

Burns and McCormack, the dashing scientists who (it won't ruin the story) save the world from imminent disaster, deliver their lines with a studied lack of irony, without so much as a wink at the camera. They battle the film's various beasts with the same let's-do-this! gusto they bring to restoring critical hard drives, rarely pausing to exercise the action hero's prerogative for jaunty banter. Moviegoers haven't seen such earnest adventurers since Claude Rains and Michael Rennie explored The Lost World.

Then there's Sir Ben as the uxorious owner of Time Safari, Inc. Outfitted with a white pompadour and poufy pinstripe suit, Kingsley seems to be channeling Zachery Smith of Lost in Space as he fawns over his clueless clients, all willing to pay exorbitant sums to bag a 65-million-year-old allosaurus.

Director and cinematographer Peter Hyams appears to have had the good sense (or natural inclination) to concentrate on CGI rather than meddle too much with the cast. So what if the actors take contradictory approaches to the material? The combination of creep and camp gives Thunder a certain frisson even as it prevents the filmmakers from taking themselves too seriously.

As for the CGI, it's ambitious but uneven. Thunder jumps straightaway into the Cretaceous, opening with Burns leading a shooting expedition into a primordial tar pit. The Discovery Channel offers more convincing dinos, but visual-effects supervisor Tim McGovern and production designer Richard Holland maintain a hothouse ambience that gives the film its prehistoric mood. Chicago, where the action takes place in the year 2055, feels a bit fossilized, although the tank-like taxis that navigate the Windy City would be a welcome improvement on the standard yellow cab.

Predictably, time travel mucks everything up, initiating a series of cybernetic tsunamis that introduce toxic hybrids into the modern world. Gigantic trees sprout through the roofs of buildings, murderous vines spear passersby with poisonous tentacles, hyperactive beetles swarm penthouse apartments.

But there's worse. Howling dragon-baboons patrol city parks in search of warm-blooded snacks, razor-toothed eels slither through the flooded subways, pterodactyl-size bats swoop down from the skies...not to mention the craven bureaucrats who refuse to allow our intrepid scientists to reactivate the quarantined time machine in order to reverse all this errant evolution.

The premise of Thunder is familiar: The slightest gesture-the famous flutter of a butterfly's wing-can have momentous repercussions at a time and place far removed from the insignificant act. Everything's connected, so people would be wise to respect life's rich tapestry. The filmmakers don't get bogged down in metaphysics, however. They barely bother with plausibility or continuity. No matter. Thunder aspires to be nothing more than what it is, an old-fashioned matinee monster movie. Save the science for The Day After Tomorrow.

-Rex Roberts