Galaxy Quest (1999)

For four years, the courageous crew of the NSEA protector - "Commander Peter Quincy Taggart" (Tim Allen), "Lt. Tawny Madison (Sigourney Weaver) and "Dr.Lazarus" (Alan Rickman) - set off on a thrilling and often dangerous mission in space...and then 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 Now, twenty years later, aliens under attack have mistaken the Galaxy Quest television transmissions for "historical documents" and beam up the crew of has-been actors to save the universe.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.

Source: TMDb