Galaxy Quest is 20 years old now, which means this genre comedy has a built-in nostalgia factor all its own. Yet that only adds to this giddy mix of nostalgia, sci-fi and TV fandom, a cleverly written movie that both pokes fun at Star Trek and embraces it. The basic premise: the cast of a TV series called “Galaxy Quest,” still appearing at fan conventions 18 years after their space opera show was cancelled, is enlisted by aliens who think the actors’ TV exploits were real, and ask them to lead an extraterrestrial mission to save the aliens’ home planet. The aliens, especially Enrico Colantoni and Missi Pyle, are a riot, and the “Galaxy” crew members – played by Tim Allen, Sigourney Weaver, Alan Rickman, Tony Shalhoub, and Daryl Mitchell – have an obvious blast spoofing their profession, and acting as… actors. Such a fun movie.