It seems in my experience that spinoff series rarely come close to the quality of their originals. Torchwood, which is a spinoff from Doctor Who, breaks that mold. It's also an anagram of Doctor Who, which is a nice hidden tribute to its parent.
Unlike Doctor Who, Torchwood is completely earthbound. It's an extra-governmental agency devoted to finding and dealing with alien threats. "The twenty-first century," says its main character, Captain Jack Harkness, "is when everything changes." It's clear by watching just a few episodes that not all those changes are going to be good.
Harkness is played by John Barrowman, a human that has been given a "gift" in the form of immortality, one that he received from Rose Tyler at the end of the first season of the rebooted Doctor Who.
Many times it isn't a gift as much as a curse--as demonstrated by the time his brother buried him alive--for nearly two millennia. Suffocating, dying, reawakening ... only to suffocate and die again, on loop, for two thousand years, would suck.
Barrowman plays the boss of a small, often contentious troop who, with their unique skillsets, help him keep control of the aliens and bad guys and whatnot that visit Earth, with a disturbingly large percentage coming to visit England and Cardiff in particular. That's because that location has a spacial-temporal "rift" which, like the ocean tides, sometimes deposits aliens and their problems and attitudes on Torchwood's doorstep. The group has an underground lair full of high-tech gadgetry, much of it ganked and reworked from those very aliens. They answer to no one, and are held in awe or contempt, depending on the government official or policeman you're talking to.
Barrowman's Harkness is the Doctor given a big shot of compassion and humanity, things the Doctor often lacks. Barrowman cleans up the messes the Doctor waves off as he boards the TARDIS to fly off to a new adventure. He sees the cost wrought by the destruction and mayhem. It adds a poignant dimension to his character, one Barrowman plays brilliantly. Captain Jack is far from perfect: he has his own issues to contend with, and sometimes they get in the way of his leadership. By and large, however, he sees sense, even if it's in the eleventh hour, and comes around.
His troop is played by some first-class actors: Eve Myles as Gwen Cooper, an ex-cop brought on board, but one who retains, fiercely, her own independence and desire to have a life outside the all-encompassing work of dealing with aliens; Burn Gorman plays Owen Harper, the resident medical doctor. Naoko Mori plays Toshiko Sato, the computer expert; and Gareth David-Lloyd plays Ianto Jones, something of a glorified concierge who becomes in later episodes romantically involved with Jack. Kai Owen plays Gwen's oftentimes estranged husband, Rhys Williams, who just can't figure out what his wife is doing, and many times has no patience for it, whatever it is.
The episodes (at least the first three seasons) are very tightly written and dramatic, and are meant for a much more adult audience than Doctor Who's. There is swearing here, and sex, and story arcs that kids probably wouldn't understand given the adult themes--existentialism, sexuality, death, and human corruptibility. I could be wrong about that. We haven't watched the final season, which was produced more as a mini-series, and was located not in England but in the U.S. It has many very mixed reviews, so we haven't bothered.
I love all the characters in this series, but I think Dr. Owen Harper is my favorite. Near the end of season three, he faces his own mortality. Tosh, who has always loved him, tries to help him escape.
It's a very moving scene, one I always dread when we cycle around to it.
If you've watched Doctor Who but haven't watched Torchwood, you are really missing out.