Earth, not space, is the final frontier of “Battlestar Galactica.”

The galaxy-wandering survivors of an apocalyptic attack begin the fourth and last season of this Sci Fi Channel wunderseries where they left off: with the faint but improbable hope that one of them has found the lost, mythical home planet.

Whether anyone should believe Kara Thrace, a k a Starbuck, the swashbuckling fighter pilot who was thought to be dead for two months and cannot account for her lost time or prove her claim, is only one of many mysteries confounding the crew of the Galactica. Another is her latest sleeping arrangements: Kara (played by Katee Sackhoff) has a husband and a lover (two, if one counts the amorous Cylon who held her captive on the robot-occupied settlement New Caprica).

“Battlestar Galactica,” which begins on Friday, is a space opera, a high-minded space odyssey with more than a touch of the daytime soaps. It is critically acclaimed and widely respected, but the science-fiction show’s fiercely dedicated cult following has become something of a mass-culture joke: the two lonely mathematicians on the CBS sitcom “The Big Bang Theory” decline lunch with a pretty girl so they can view the commentary on the DVD of Season 2.

And that is a disservice because “Battlestar Galactica” is one of the more beguiling series on television, an action-adventure drama that travels through time and space to explore morality, politics and metaphysics. Science fiction often serves as a modesty curtain that permits authors to think big thoughts at a safe remove  special effects and laser make-believe palliate abstract musings and pompous parables that might otherwise bore or offend viewers. (Without phasers and Vulcan death grips, the moralizing streak in the original “Star Trek” would have been insufferable.)