Alas, I don't believe this new, limited "Never Give Up, Never Surrender" collectors' edition is any different from the original Blu-ray release, other than its cool package, but it's a pretty good Blu-ray, so if you don't have it already, it's worth having.
Tim Allen plays an actor who performed the role of Commander Taggart, the "Captain Kirk" of an "Enterprise-like" starship on a long since cancelled weekly TV show. Sigourney Weaver is his communications chief whose chief function on the show is decorative; Alan Rickman is the ship's alien doctor – and he hates that fact; Tony Shalhoub is the ship's (non-Scottish) engineer, and Daryl Mitchell is the whiz kid helmsman – now all grown up.
In the "reality" of the film, however, they're a bunch of washed up actors who make most of their livings these days by showing up at "Galaxy Quest" conventions and tiredly going through the motions while charging fans for their autographs.
It's at one of these conventions that a group of real aliens shows up. Naturally, Nesmith/Taggart writes them off as just a few "GQ" fans from the more lunatic fringe – or, to be fair to fans, the more "into it" "cosplay" fans. But no, these aliens are Thermians, and they're for real, and they're begging Allen for his help in their desperate conflict with a race of evil, crab/lizard-like aliens.