For sci fi, it’s smokin’

SEX and science fiction go together like dilithium crystals and warp drives. But “Torchwood,” returning this week for a second season on BBC America, makes a point of it: The universe runs on desire. A sci-fi soap opera about talented outside-the-government investigators policing a space-time rift -- in Cardiff, Wales, as good a place as any -- it belongs to the same fictional universe as creator Russell T. Davies’ reborn “Doctor Who”: grown-up, dark, energetic and sometimes hard to keep track of. But it feels good to let it flow over you.

Running the show is Capt. Jack Harkness (dashing John Barrowman), an immortal 51st century “time agent” who disappeared at the end of Season 1 to spend a few episodes in Season 3 of “Doctor Who,” working out his priorities. But he’s back to “Torchwood” -- the name of the organization and an anagram of “Doctor Who” -- in time for the opening of Season 2. Coming from a world beyond our terrestrial, gender-based partner preferences, Jack finds mere heterosexuality/homosexuality “quaint”; his 21st century cohorts don’t put too fine a point on those distinctions either.

As the story of a somewhat incestuous Scooby Gang working in a city that serves as a gateway for bad things from afar, “Torchwood” is very much in the mode of “Buffy the Vampire Slayer” -- which makes the season-opener guest appearance of James Marsters, who was Spike in “Buffy,” appropriate and, for the right kind of nerd, exciting. Marsters plays an old partner of Jack, in time-agenting and beyond -- and I would not dream of saying more.

(BBC America, Sat., 6 and 9 p.m.)

-- Robert Lloyd