The success of "Stargate SG-1" has always been a head-scratcher.

Really, there’s little difference in storytelling or production quality between this show and, say, "First Wave," "Forever Knight," "The PSI Factor," "Andromeda" or any of the other competent, made-in-Canada mediocrities that fill up the cable dial.

And yet "Stargate SG-1" boasts an international following of 17 million weekly viewers in 64 countries. It’s entering its eighth year of production, meaning it’s lasted longer than "Star Trek: The Next Generation."

Now the show’s producers hope to parlay that popularity into a spinoff, starting Friday on the Sci Fi Channel. "Stargate Altantis" sends a team of intrepid Earthling explorers - via the stargate, of course - to an underwater city in an unexplored galaxy so distant there may be fuel for only a one-way trip. Yes, that sounds like a dumb scenario, but silliness has never been much of a barrier for science fiction.

Actually, the secret to "Stargate SG-1’s" appeal can be reduced to its attractive cast and the chemistry among them. All of the regulars are good-looking actors who play well off each other; they’re fun to spend time with. But the most important is career military man Jack O’Neill, played with great wit and subtlety by "MacGyver" himself, Richard Dean Anderson.

Without the light touch and intentional humor the charismatic Anderson brings to the table, "Stargate Atlantis" will have little chance to build on the good will earned by its parent show. Fortunately, the producers seem aware of this, which means the cast of the new show is not only attractive but also capable of delivering a funny line now and then.

Leading the way is Joe Flanigan as Major John Sheppard, a headstrong smart-aleck who can always be counted on to go against orders when it’s the right thing to do. Looking like a cross between a young Jimmy Stewart and a young Alec Baldwin, Flanigan does a surprisingly good job of importing Anderson’s style of heroics and comedy to the new scenario.

Torri Higginson plays Dr. Elizabeth Weir, the scientist leading the mission, while Rachel Luttrell plays Teyla, the obligatory hot-but-really-smart alien babe whose role it is to help out the Earthlings, offer frequent rescue opportunities, and flirt with Flanigan’s fly-boy charmer.

The plot of the two-hour pilot is little more than the set-up for the series to follow, and its details are of negligible consequence, since an inventive mythology has never been the strongest element of the Stargate universe. As long-time viewers of the original series know, an alien outpost found in Antarctica may be a remnant of the "Ancients" who seeded the universe with stargates and populated it with humanoids.

Turns out the Ancients were also the builders of the lost city of Atlantis, which they could submerge for safety when threatened by their enemies. When Earth was attacked, they moved lock, stock and force field to a planet far, far away.

Gazillions of years later, the "Stargate Atlantis" team accidentally powers up the now-abandoned city, draining the force field and endangering the mission. Sent on a hasty quest for more fuel, Sheppard encounters Teyla and her people, a zesty race of primitives preyed upon by the nasty, vampire-like Wraiths.

So by the end of the pilot, our heroes have new friends, new enemies and endless opportunities for fresh adventuring. Will they vanquish the Wraiths, lead Teyla’s people to the freedom and prosperity of a modern consumer society, and find enough go-juice to get home to Earth?

Not, with any luck, for at least seven years.

STARGATE ATLANTIS

Stars: Joe Flanigan, Torri Higginson, Rachel Luttrell, David Hewlett, Rainbow Sun Francks

Airs: 8 p.m. Fridays on Sci Fi; two-hour premiere this week at 7 (preceded at 6 by an encore of "Stargate SG-1’s" eighth season opener, with minor crossover plot points)