What would happen if William Shatner, Leonard Nimoy, DeForest Kelley and the rest of the original Star Trek cast were suddenly beamed aboard the real USS Enterprise and expected to save the galaxy, a la Captain James T. Kirk, Spock, "Bones" McCoy and all the rest of the merry crew?

We'll never know. But you can get a good sense of the answer by watching Galaxy Quest, which sends up Star Trek and its signature cast and characters in grade-A style.

Tim Allen is Jason Nesmith, an aging and arrogant actor who is still very much caught up in his heroic role as Commander Peter Quincy Taggart. He basks in the idolatry of his fans while having no real clue that his co-stars don't like him very much. But still, they all get together to work the crowds at massive science fiction conventions, where they maintain a huge following even though their cheesy, low-budget show was cancelled long ago.

But things turn wacky when a group of real aliens comes seeking aid. They've intercepted the old Galaxy Quest episodes in space and believe them to be historical documentaries about the exploits of actual heroes. Faced with a menacing lizard-like alien, they realize that only Commander Taggart and his bold crew of the Starship Protector can save them. So they create a starship to the specs of the show and whisk their would-be saviors away into space.

Joining Allen are Sigourney Weaver as actor Gwen DeMarco and character Lt. Tawny Madison, the show's cleavage factor; Alan Rickman as sneering British stage actor Alexander Dane, who loves and despises his role as the alien Dr. Lazarus of Tev'Meck; Tony Shalhoub, whose engineering genius as Tech Sgt. Chen has no relationship to his real personality as laidback actor Fred Kwan; and Daryl Mitchell as actor Tommy Webber, who started in the series as child prodigy Lt. Laredo. At the convention preceding the events of this movie, they are joined by a crewman extra named Guy (no last name), whose role in the series was to be killed before the first commercial. Sam Rockwell plays Guy with great anxiety, since he's convinced his fate is sealed in this real adventure.

It's fun to watch the actors adapt to their situation and begin responding to events as their alter egos would. The commander becomes a bold leader, flinging himself into peril without regard to his own safety or the integrity of his shirt. DeMarco loses most of her shirt, too, as she follows her leader through danger, even as she berates the writers who filled their ship with such nonsensical hazards simply for the sake of visual effect. Dane learns that the lines he knows by rote can actually inspire those around him. And Kwan discovers what a woman with tentacles can really do.

It's even better when Nesmith manages to communicate with a horde of rabid Galaxy Quest fans back on Earth, tapping into their obsessive knowledge of the show to get through a few rough patches.

Standouts among the helpless aliens are Enrico Colantoni as Mathesar, who refuses to believe that Nesmith and his crew are anything less than their heroic counterparts, and Missi Pyle as the tech-smitten Laliari.

Visually, the film makes marvelous use of some skimpy '60s-style props and sets among the much more "real" effects of genuine outer space and alien encounters.

Galaxy Quest is a silly film, but it's silly in a way that will stick with you. If you've lived in a shell and are unfamiliar with Star Trek, its cast and crew, you might miss some of the obvious jokes in this one. Otherwise, you'll have a blast and quite a few hearty laughs as Allen and crew lovingly poke holes in the Star Trek tapestry.