Lena Headey, star of Fox's new Terminator: The Sarah Connor Chronicles, looks like someone you don't want to mess with. In a gloomy underground tunnel in Los Angeles, she and two burly actors in police uniforms are shooting a fight sequence so intense they're stirring up clouds of dust. Headey, clad entirely in black, kicks and jabs until she has the cops on the ground.

"Lena can summon an internal fierceness that is incredibly important for this role. There's a fire in her," consulting producer James Middleton says. And this morning's brawl is just her warm-up. Later today, she'll rappel down an elevator shaft — after the crew blasts a giant hole in the wall.

The show picks up two years after the 1991 film Terminator 2: Judgment Day left off. Living under the radar and on the run with her teenaged son, John (Thomas Dekker, who played Zach on Heroes), Sarah's trying to train him to be the savior of mankind, protect him from the Terminators sent from the future to kill him, and stop the evil computer network Skynet from starting a nuclear apocalypse. And oh yeah, get dinner on the table and make sure the kid does his homework. "You are looking at someone who's trying to be a great mom and give her kid normality, and at the same time they're both aware that their day job is to save the world," Headey says. 

Through it all, she's pursued by FBI agent James Ellison (Richard T. Jones). The question of whom Sarah can trust will dog her as she tries to find out how Skynet became operational — considering in T2 she blew up the Cyberdyne lab where it was being built. She and John will get some help from Cameron (The Unit's Summer Glau) — named after franchise creator James Cameron, who is not involved in the TV series. Like Arnold Schwarzenegger in T2, she's a Terminator sent to protect John.

But a killer machine, played by Owain Yeoman (The Nine) in the first episode, gets in their way. He's blown apart but puts himself back together, transforming his appearance so drastically that he'll later be portrayed by a different actor, Deadwood's Garret Dillahunt.

There are so many blasts, crashes and shoot-outs that die-hard Terminator fans may want to install surround-sound just to maximize the viewing experience. No pale imitation of the original, this small-screen show has big-screen production values and a not-so-little budget. ("The series is expensive, but not the most expensive out there" is as specific as Middleton will get about costs.) It also benefits from the expertise of stunt coordinator Joel Kramer, who served in the same capacity on T2.

The physical demands of the job suit Headey, who played Queen Gorgo in 300 last year. "I've always loved keeping fit," says the 34-year-old British actress, her slim frame tucked into a director's chair during a break. Some of her fighting ability comes from training in a gritty south London boxing gym. "It gives you great concentration, which I don't possess naturally," she says.

But there will be quieter moments in the show. "It's a different time," Headey adds. "We get to know Sarah more. I think she was a closed door emotionally in the movies, and here you're getting to see her and John [interact more]." We also may see more of her love life with Charley Dixon (Dean Winters), her fiancé she left behind in the premiere. But as Middleton points out, "People who get close to her have a high mortality rate."

For more on Sarah Connor, pick up the new issue of TV Guide, on sale now.