Galaxy Quest (1999)

The cast of an old 1980s space-adventure television series, Galaxy Quest, spend most of their days attending fan conventions and making promotional appearances. However, they are recruited by an alien race called the Thermians to help defend both Earth and the Thermians from a reptilian warlord named Roth'h'Har Sarris. The cast, thinking it's just another job, joins forces with the Thermians and embarks on a real space adventure. They must learn to work together and use their skills to overcome challenges and defeat Sarris. In the end, they succeed and the cast members are hailed as heroes. The experience inspires a sequel series and the universe of Galaxy Quest expands.