Galaxy Quest

Galaxy Quest (1999)

For four years, the courageous crew of the NSEA Protector — Commander Peter Quincy Taggart, Lieutenant Tawny Madison, and Doctor Lazarus — set off on a thrilling and often dangerous mission in space... until their series was cancelled! Now, twenty years later, aliens under attack have mistaken the Galaxy Quest television transmissions for "historical documents" and beam up the crew of has-been actors to save the universe. With no script, no director, and no clue, the actors must turn in the performances of their lives.

Synopsis via TMDB