Deep within the walls of a garishly ornate faux-Gothic mansion in Chatsworth, Calif.—the porn capital of the world—an elaborate chandelier rocks overhead as a group of grim-faced Torchwood operatives rushes through the cavernous hallway, an aura of urgency and dread filling the air. A crime has occurred, and forensics agents and black-suited government types swarm at the edges of the scene.

In the nearly triple-degree heat, actor John Barrowman removes Captain Jack Harkness’s trademark long coat, draping it over the furniture as he whispers conspiratorially to Torchwood’s newest members, Rex Matheson (Mekhi Phifer) and Esther Drummond (Alexa Havins). Elsewhere, Eve Myles—who plays Jack’s plucky trusted associate, Gwen Cooper—is also dressed inappropriately, her black leather jacket vastly at odds with the swelling summer heat.

“We’re a long way from Cardiff,” says Myles about moving the show’s production from Wales to Los Angeles for much of Season 4, entitled Torchwood: Miracle Day. “It’s awesome lush.” And, just like that, we’re back in Cardiff again.

Torchwood, a spinoff of the venerable sci-fi drama Doctor Who and a cult favorite in Britain and the States, is shooting its eighth episode of its fourth season, which recounts the global panic that follows the so-called Miracle Day, when death ceases to exist on the planet.

“Torchwood always asks the questions that no other television show will ever touch,” says Barrowman, who plays the sexually voracious, immortal time traveler Jack Harkness, during a hair and makeup touch-up. (The key to his character’s spiky hair? A flatiron between shots.) “It asks, what would we do as a society if this happened? How would we behave? How would we live our daily lives? Jack’s immortality has always been that he dies, he comes back to life, and he just gets on with it. It’s a troubled case for him, but how humanity deals with it is an entirely different thing.”

The 10-episode season, which begins Friday on Starz, finds Torchwood’s Captain Jack Harkness, Gwen Cooper, and their new associates attempting to get to the bottom of the miracle, and, as the action slingshots from Washington, D.C., to Los Angeles and from Shanghai to Swansea, Wales, the world becomes a darker, more dangerous place, as the definition of death is turned on its head.

Torchwood: Miracle Day is an outgrowth of the change in format that occurred during the show’s landmark third season (subtitled Children of Earth), a five-episode miniseries that aired across five subsequent nights in 2009 on BBC One and BBC America in the U.S. Gone was the episodic monster-of-the-week format; in its place was a taut, serialized single narrative that balanced the show’s winning combination of sex, violence, darkness, and humor. Time magazine’s James Poniewozik called Torchwood: Children of Earth, which depicted some very damaged individuals making some hard choices to save the world, “an unmissable event,” writing, “It's a story about terrible things, but also about how decent people can be driven to do them.”

Critics and audiences largely raved, leading creator Russell T. Davies to pursue a fourth season, one that would end up being a coproduction among BBC Worldwide Productions, BBC One, and pay cable network Starz in the U.S. (It was originally set up at Fox, but the network passed on the project.) The central conceit: a world where death ceases to exist.

“It was one of those tortured stories that’s been in my head for a long time,” says Davies, over breakfast at 3 Square Café in Venice, Calif. “Death Takes a Holiday. It’s an old idea … and a variation on zombies, but we don’t follow that particularly. Why do that when The Walking Dead is on the air?”

A few weeks after Torchwood has wrapped its production, Davies nurses a coffee on an unseasonably gray day, the antithesis of the sunshine and heat of that Torchwood location shoot, much more similar to Swansea, Davies’s childhood home where he has recently bought a house.

“With Torchwood, I’ve tried to say this isn’t a metaphor for the human race; this is the human race,” he says. “It’s literally us, what we would do. By episode nine, you have a rebuilt society, the whole world—certainly the Western world—has changed. It operates differently. There’s a massive recession, the pensions market goes under, which causes a domino effect. You actually see people living different lives.”

Likewise, Myles ponders whether society would embrace a miracle such as this one.

“Your initial thought would be, wow, wonderful, I’ll never outlive my daughter and she’ll never outlive me,” she says. “We’ll never have to fear death again. But the reality of that is that this planet doesn’t have the resources for that continuous life to carry on ... Soon, the world starts imploding and has become unrecognizable. It’s become a prison, and it’s horrific.”

The central mystery within Torchwood: Miracle Day is just who or what is behind the sudden miracle that has erased death and kicked open a hornet’s nest of philosophical and religious questions, issues of morality and theology. Is there an afterlife? Is there any chance for divine retribution? Bill Pullman’s child killer, Oswald Danes, serves as proof that the universe is out of whack; after his execution by lethal injection goes awry, he sums up the transformative abilities of the miracle, becoming a lightning rod within the media. As for the villains, they’re not in plain sight.

“We’ve got the biggest monster we’ve ever had before,” says Myles. “And it’s not in a glass cage, it doesn’t have three heads, and it doesn’t spurt green blood.”

Davies agrees. “The villains are invisible,” he says. “They have to be with this sort of science fiction. If they were visible, if there was someone in a boardroom plotting in episode one, Torchwood, the CIA, and Interpol would just look stupid … Even the villains are looking for who’s behind it. [Lauren Ambrose’s mercenary PR executive] Jilly starts looking; Oswald starts looking. Torchwood's chasing the ghost, and the villains are chasing the ghost.”

Is a nebulous multinational pharmaceutical manufacturer behind the inexplicable erasure of mortality? Is this “miracle” more of a curse than a blessing, and how does humanity cope with the knowledge that their lives have become eternal? Just how thin is the construct of society?

These aren’t questions that typically pop up on, say, Doctor Who, but Davies maintains that Torchwood itself is a reaction to Who, where he served as a head writer and executive producer for five years. This is a much more adult drama with a mature sensibility about violence, morality, and sexuality, one that hasn’t changed with the move to America. “We’ve got a [male] character that has passionate sex with men on television,” says Barrowman. “Not a lot of other shows will show that; they won’t even show them kissing.”

But fans needn’t have seen Davies’s run on Doctor Who nor the first three seasons of Torchwood in order to jump into Miracle Day. Havins, who plays CIA watch analyst Esther Drummond, says her character is intentionally asking questions that newbies at home might be.

“Esther is discovering things along with the audience, asking, ‘What is Torchwood?’” Havins says. “It has a little bit of everything for everyone. If you like action and explosions, we’ve got that for you. If you’re into romance, there’s a sprinkling of that there. And there’s always some levity.”

Whether Torchwood can find a mass American audience on Starz remains to be seen. The channel, now under the leadership of Chris Albrecht, has had success with Spartacus but recently canceled first-year drama Camelot. As far as the future of the franchise, Davies has no plans at the moment, other than the fact that should the ratings be there, Torchwood will be back for another single narrative season.

“If the next thing I come up with is a monster in an attic,” he says, “you will get 10 weeks set in an attic. But I’m not about to do that. The shape of the story dictates what you write. It’s not prescriptive. It’s not saying, let’s fly to Honolulu and find a Honolulu monster. I do have vague ideas about what our next story could be. It would be very different but similar to Miracle Day. And if Torchwood doesn’t come back, I’ll tell a different story.”

Still, Torchwood is always about changing or dying, it seems.

“You have to change,” says Barrowman. “With Torchwood, we’re always changing, we’ve always been a chameleon that has changed with the times. We never do the same thing twice and that is what people have come to expect.”

“But we still have the true heart of Captain Jack and Gwen Cooper, and there’s still excitement, adventure,” he says, laughing cheekily, “and lots of sex.”