The Adventures of Stella Star (1978)

Outlaw smugglers Stella Star and Akton manage to pick up a castaway while running from the authorities, who turns out to be the only survivor from a secret mission to destroy a mysterious superweapon designed by the evil Count Zarth Arn. The smugglers are soon recruited by the Emperor of the Galaxy to complete the mission, as well as to rescue the Emperor's son, who has gone missing.