As squadron-loads of RAF Tornados are scrambled for a dramatic dogfight with a super-sleek UFO, there is more hi-tech hardware on display in the first 10 minutes of Invasion: Earth, BBC1's new big-budget sci- fi series, than in the entire runs of Dr Who and Blake's Seven put together.

Weighing in at a reported pounds 1m per episode, Invasion: Earth puts the era of wobbly sets in Surrey sandpits and gizmos constructed out of egg-boxes and loo-rolls firmly in the past. We are now in the age of Men in Black, rather than BBC TV's more traditional Men in Bacofoil.

Jed Mercurio, the writer and co-producer of Invasion: Earth was determined to consign those charming days of low budgets and even lower production values to history. He sighs at the memory of Britain's lo-tech sci-fi shows of the 1970s. "I used to watch Blake's Seven and think, `Why does this spaceship have a blue line around it when the Starship Enterprise in Star Trek never does?' That used to annoy me. Nowadays you can't broadcast dodgy special effects and then put up a caption saying, `Sorry, this is what the budget was'. You have to do it with high production values because the audience has been spoilt by the special effects on things like The X Files and Independence Day."

Fortunately, with the rapidity of advances in technology, special effects are increasingly coming into the BBC's price-range. The creation of an exciting air-battle or a convincing-looking alien race is no longer going to break the bank. "Special effects are becoming more and more affordable," says Mercurio, "and looking more and more like the real thing. In the past, if they wanted to make a drama about the Navy, producers would say, `Oh no, we've got to go to sea.' Now you can computer-generate all that. Thanks to Mr Cameron [the director of the special-effects-heavy Titanic], you can download a lot of that technology from the Internet for tuppence- ha'penny. Special effects are going to become more and more an integral part of programme-making."

Mercurio, a former hospital doctor in Wolverhampton, made his name with the mould-breaking medical series, Cardiac Arrest, but now his vision extends beyond doctors and nurses. "Sci-fi gives you the scope to do grand stories. Now the Cold War's over, most people's experience of life and death is not from wars but from hospitals, and I've done that already. Sci-fi is now the only way of doing stories about the future of mankind. As we approach the millennium, it looks like World War II is as big an event as we are going to get. Now the most likely scenario for conflict is in sci-fi. It's one of the few forums where we can tackle stories on a major scale."

Fred Ward, the American star of Short Cuts and The Right Stuff, who plays a Nato commander in Invasion: Earth, also sees sci-fi as a symbolic way of discussing more immediate concerns in our lives. "In the 1950s, sci- fi was a metaphor for the Red scare. Today it could be a metaphor for a virus that's killing the world - Ebola or HIV or bacteria that is immune to antibiotics... Maybe this is a way to get that out in our fantasies."

With its grand themes and matching production values, Invasion: Earth is a television programme that will certainly grab headlines - but then Mercurio is well used to that experience. The "raunchiness" of Cardiac Arrest got the popular press very hot under the collar. "The tabloids latch onto sex in any form because it sells newspapers. But one doctor friend of mine said: `The problem with the series is that it doesn't have nearly as much shagging as actually goes on.'

"Other medical programmes have gone for that classic `Will they, won't they?' romance between middle-aged people. But the reality is that if you've got a lot of young people working long hours together, they're going to fancy each other and have sexual intercourse. That's not telling anybody anything they didn't know already."

With its uncompromising portrayal of a National Health Service creaking at the seams, Cardiac Arrest also had right-wing politicians up in arms. "It doesn't take much to outrage right-wingers, but I was still taken aback by the response. What I wrote about was my experiences as a junior hospital doctor. Some doctors had far worse experiences - some even killed themselves. Certain people in politics and the press felt there was a political spin to Cardiac Arrest, but there was no political agenda to what I was doing. I was just making a programme which people felt had a truth and an attitude that other medical dramas didn't. My starting- point was, `Would I watch it?'

"Now, of course, I'd be over the moon about that sort of attention," he laughs. "It got the programme talked about. So thank you very much, Virginia Bottomley."

`Invasion: Earth' is on Friday at 9.30pm on BBC1