Jane Espenson

When Jane Espenson was first approached about joining the writing staff of Torchwood, the veteran television writer whose credits include Buffy the Vampire Slayer and the rebooted Battlestar Galactica had just one thought: "It’s going to be hard to get stakes higher than aliens wanting to gobble up the children of Earth."

Fortunately for fans waiting to see how immortal alien hunter Captain Jack Harkness would return after suffering the devastating losses he did in 2009’s Torchwood: Children of Earth, series creator Russell T. Davies had just the ante-upping answer. For the U.K. hit’s fourth season, (premiering Friday at 10/9c on Starz), he’d move the action from Cardiff, Wales across the pond (and into the bigger-budget land of pay cable), transform actor Bill Pullman into a worldly "monster," and hit Jack with a head-spinning reality in which everyone is cursed with living forever — except, suddenly, him.

Watch the trailer for Torchwood: Miracle Day

"Immortality sounds fantastic for about a second," says John Barrowman, who has played the unsinkable Jack since the sci-fi show’s 2006 launch on BBC Three. "Then everyone begins to realize what it means for humanity, for the population. The ethical questions come up. For some, it’s a real disaster. You get in a car accident? You’re mangled? You’re still alive when you should be dead."

"When I first heard about this," says Eve Myles, who plays Torchwood’s only other remaining member, Gwen Cooper, "I was seven, nearly eight months pregnant, and my waters nearly went. I nearly had the baby when I found out the plot."

The Torchwood faithful will be comforted to know the 10-episode fourth season, Torchwood: Miracle Day, is not a reboot for U.S. audiences. (New cast member Mekhi Phifer prefers "Torchwood on steroids.") Instead, the story picks up several months after a tormented Jack left the planet, leaving behind a pregnant Gwen and her husband Rhys (Kai Owen). Jack is compelled back when, simultaneously, people stop dying and Gwen, now a mom in hiding, comes under attack. It so happens she’s aching to get back into world-saving mode. "She is, literally, baby in left arm, magnum in the right," Myles says. "Her big challenge this year is juggling Torchwood and being a mum."

For the unfamiliar, Torchwood began its life as a more ballsy monster-of-the-week spin-off of Doctor Who following a covert agency defending the world from alien danger; all but two of the Torchwood’s team members have been killed, and the group’s leader, Jack, has demonstrated a voracious (if little-seen), omnisexual (men, women, alien) appetite. Davies decided to make the show’s third season a taut, five-hour thriller about a single case: an otherworldly threat to the Earth’s children. Two years later, Miracle Day follows the same model — though it consists of 10 hours — and the threat is eternal life.

Torchwood: Miracle Day: What happens if everyone is immortal?

"It’s one of those ideas that’s been ticking away in the back of my mind for many years," Davies says. "It could have been a Doctor Who episode [the show he revived in 2005], but I thought it was better as an interesting way to present Torchwood to both new and old viewers. Jack’s immortality is fascinating, but it could be hard to introduce to a new audience. You don’t want to roll back four years and say he’s immortal because he was exterminated by the Daleks in a completely different series! We came to pole vault over that problem by giving it to the entire world. I wish I could tell you it was planned. It was pure accident."