Click here to watch the trailer for The Sarah Connor Chronicles

Bad sci-fi is possibly the worst form of fiction there is; and average sci-fi is fine for fans. Good sci-fi, however, can take the vague, floating fear of a nervous generation and focus it on one relentlessly magnified version of their terror until it becomes impossible to turn the gaze away. War of the Worlds managed it twice (not including the Tom Cruise version, obviously); 2001: A Space Odyssey had its moments. But for sheer contemporary horror, the first Terminator movie should be in the top five of any list worth its salt.

Released in 1984, in the final act of the cold war, its vague hints of imminent nuclear holocaust, coupled with the unstoppable march of Arnold Schwarzenegger’s killing machine, made it a genuinely alarming futuristic take on the serial-killer flick. Later films swapped paranoia for CGI and became little more than funfair rides. Schwarzenegger was woeful as the good ’bot, leading to fruitless time-travel speculation that going back to 1985 to persuade him to run for office then might have spared the world Twins and Kindergarten Cop.

The incomprehensible Terminator 3 appeared to finish off the wounded franchise, until Terminator Salvation was announced. That clanks onto the big screen in 2009, with Christian Bale as John Connor – who was, I thought, killed at the end of T3. But that’s the other beauty of sci-fi: you can do whatever the hell you want.

Fortunately, this anything-goes philosophy can produce results – such as the forthcoming television spin-off, Terminator: The Sarah Connor Chronicles. The producer, James Middleton, who is involved in Salvation as well, says the television series is an “alternative reality”. “It’s a different possible sequel to T2, in which the events of T3 haven’t happened and won’t happen,” he explains, with the kind of fervour that alarms pretty girls when they meet engineering students at parties. “Sarah Connor and John have, they hope, damaged the chances of the robots winning, but they are on the run from the law and any possible attack from the future.”

Despite this rather tortured pitch, The Sarah Connor Chronicles is good. This is down to a few things. First, the undeniable talent of the three leads. The British star of 300 and St Trinian’s, Lena Headey is an intense, emotional Sarah Connor; Thomas Dekker (Zach from Heroes, my dear nerds) slouches as a sulkily adolescent John Connor; and the newbie Summer Glau manages to smoulder with sensual promise as an ice-cold killer ’droid from the future. Which is a tough gig.

The show also benefits from being on the small screen. For a start, the budgets don’t allow for an endless series of technologically improved wibbly metal bits from the future. We are back to the first machine: a robot covered in skin that can change its voice and marches on unscathed through flames, walls and speeding trucks.

Television being as it is these days, you can’t relax in the way you can with a Christian Bale movie, confident he isn’t going to be offed in reel one for that kind of money. In TV-land, they kill their heroes. Nobody survives two seasons of 24; Glenn Close’s legal thriller, Damages, begins with almost everyone dead; and Spooks regularly takes out its cast, including its first real star, Holby City’s Lisa Faulkner, halfway through its second episode. So, as the mechanical murderer strides towards John Connor, sprawled in his high-school car park with no visible means of escape, the thought flicks through your mind: “It’s called The Sarah Connor Chronicles. He might not live through this one.”

And, finally, the series has a heart. Whereas, in T2, Sarah’s interaction with John basically consists of screaming “Run, John”, the mother/son relationship in Chronicles is complex and moving. She wants to keep him safe, but to do so, she becomes a drill sergeant, not a mum. She can see he’s drifting away from her, but she has to keep screaming to keep him alive. Like Sigourney Weaver in the Alien films, Headey’s Sarah is a lioness defending her young, fully aware that the legends from the future carry no mention of her name.

Headey handles it beautifully, projecting heartbreak, fear and determination in a single glance. There is an almost Shakespearian battle between fate and character in the early episodes, which she manages to play while delivering a spinning back kick and letting fly with a hefty .45. Some British papers have accused the actress of “not measuring up” to Linda Hamilton’s muscular physique in T2. This story is largely based on afan site called the Sarah Connor Charm School, which ran pictures comparing Headey’s and Hamilton’s back muscles – an exercise that ranks pretty high on the list of Most Futile Uses of a Vast Global Computer Network and suggests that, when the robots do turn against us, it will be because we are using their complex circuits for exactly that kind of nonsense.

Face to face with Headey, I have to say I wouldn’t fancy the average journalist’s chances if it came to a scrap. More wiry than skinny, the 34-year-old sports six tattoos – she’s wearing a white singlet, so three are visible; the rest are “elsewhere” – and the kind of muscle definition that proves her time at a south London boxing gym was well spent. Middleton says that she warms up for action scenes by sparring with a former US marine at the side of the set. Indeed, on this cold, jet-lagged day, it’s talk of boxing that gets her beaming.

“I’m kind of jabby and kind of hooky,” she grins. “I’m pretty fast, and I’m strong. Boxing’s about concentration, focus and determination, which you realise when you practise it. It’s not about brute strength. I’m addicted to it.”

The Hamilton v Headey row doesn’t interest her (although put them in a south London car park at closing time and I know who my money would be on). While she was aware of the films when she was growing up, she made a conscious attempt not to watch them in preparation for the role. “I listened to a lot of them,” she smiles from beneath her floppy fringe. “So I knew the timeline and content of everything, but I didn’t want to deliver a copycat performance. I mean, you couldn’t get more unSarah Connor than a Brit from Yorkshire. I was a teenager when the films came out. So people say, ‘You weren’t a fan?’ Well, I had other things going on. Oh, there’s shoes, and there’s a boy, and there’s Terminator. You know – it was a busy world.”

She plays Sarah with “a primal instinct to protect someone you love that’s tangible”. She also thinks that, at heart, Sarah is crazy, although her son keeps her sane. In a curious life/art twist – a sentence you don’t often get with killer-robot shows – she finds her co-star Dekker’s Welsh roots have the same effect on her. “I enjoy comedy, and I enjoy laughing, and I see the irony in lots of things, which is troublesome in sci-fi sometimes.” She gives a quick smirk. “I find it difficult when I have to explain scientific things, and I always say, ‘Oh, I think Thomas should say it.’ He does it so well, and his sense of humour is absolutely wicked, so he gets my double entendres.”

She is happy that the ratings are good in America, and hopes that the end of the writers’ strike will allow her to explore more humour in the character. Which opens up the final definition of sci-fi: sci-fi that can laugh at itself. It’s rare, and it has to be done with affection, but if you can pull it off, sci-fi with wit – plus fit naked bodies, explosions, persecution, time travel, fear and killer robots – that sort of sci-fi can be great.

Terminator: The Sarah Connor Chronicles begins on Virgin 1 on Thursday

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