Remember the good old days when all we had to worry about was the British Empire's decline? 

Tomorrow's Worlds (BBC2) is a brilliant series, looking at the history of science fiction. This week, it focused on 'invasion' and how sci-fi authors and directors have interpreted that theme.

The programme showed that fear of invasion has haunted us since the late nineteenth century, when imperial Britain first got an inkling that its vast Empire wasn't as mighty as it used to be, with wars, restless natives and rising industrial competitors sending ripples of insecurity through the country.

HG Wells captured this mood in The War of The Worlds, where English suburbia is invaded by the Martians. Gentility and prosperity are hopeless against the invaders. Presenter Dominic Sandbrook gives the novel its due as the tripod-stamping granddaddy of them all, setting the tone for the turbulent, fearful 20th century to come.

For the century was all about fear of invasion, whether that was from Zeppelin raids, the Nazis crossing the channel, of the Cold War with the spread of communism, or the appalling threat of nuclear war when observers watched the skies for incoming bombers.

Sci-fi has been there to capture these fears and create great stories and films from them, and the beauty of sci fi is that no topic is too crazy. Whatever seems most outlandish and weird - tripods, walking plants, psychic children with glowing eyes, or monsters dragged up from the sea - can all be boiled down to reveal the salty sediment of truth. Those Martians stalking across Surrey were really about the decline of the British Empire; The Invasion of the Body Snatchers was really about Cold War paranoia; Godzilla was really about the terrible misuse of science. Within the weirdness is a lesson.

But despite the subject matter of threat and invasion, and despite the constant clips showing death and destruction, the programme had an enjoyable, jaunty tone. The talking heads who featured all had glee in their eyes, speaking of a subject they loved and which gave them the artistic freedom to leap and imagine and terrify. And they were all experts, rather the usual chattering celebs. John Landis appeared, a man who never seems to age. Neil Gaiman, Steven Moffat, John Carpenter and David Tennant appeared too, and the undercurrent to their contributions was delight, especially from Tennant when he spoke of how wonderful it is to act opposite a Dalek. The monstrous thing is physically present on the set. The Doctor isn't required to speak his lines to someone off-camera, with the Dalek added in later by the special effects team. They really are wheeled in to star opposite him.

They communicated their delight and the viewers could share that. We like to be scared, as long as we're being scared from the couch with a book or the remote control or a bag of popcorn in our hands. Science fiction allows us to indulge and process our fears, with the very spectacle pushing it away from the humdrum world we live in, making it all seem fantastic and improbable. And yet…

But obviously not all visions of the future spark an enjoyable fear. Consider Threads, a film which doesn't thrill. Instead, it makes you quietly ill with dread. Threads wasn't mentioned in this episode, being more 'speculative fiction' than 'science fiction', but it could have been slotted in because isn't a nuclear attack the ultimate invasion? But, on second thoughts, had they included Threads it would have dented the boyish, gleeful tone of the programme. No-one could talk of that particular film with a chuckle.