I blame Shakespeare. Ever since the Prince of Denmark, fresh from communing with his father's shade, slapped his sceptical friend down with the words "There are more things in heaven and earth, Horatio, / Than are dreamt of in your philosophy", scriptwriters have had a ready-made wise cliche available to justify their forays into the occult and the supernatural.

Otherwise, of course, we would not be giving houseroom (let alone prime airtime on one of our main TV channels) to a drama series based on the presumption that every barmy urban myth you've ever heard, every millenarian fantasy that any mad sect has ever devised, and every bit of New Age crystal- ley-healing that any bearded psycho-quack has ever peddled are all true. Not only true, indeed, but suppressed by our evil rulers, for reasons of their own.

But this is what The X Files (BBC1, Thursday) are all about, and we cannot get enough of them. This week Agent Scully (the redhead) returned for another series searching for Agent Mulder (the brunet), who was last seen being burnt alive in a buried box-car with two score government-cyanided ETs. Could he possibly have survived? As soon as you knew that the box- car was in a quarry, which was in turn part of a Navajo reservation, you had a good feeling about how it would turn out. Recovered from underneath some surprisingly light rocks, a week in a sweat-lodge, naked and covered in green leaves, being chanted over by Walks-on-Water, brought Mulder back from the brink.

While he teetered on the edge of death, surrounded by stars and spinach, his late father shared wisdoms with him ("Search for the truth that you will find if you are to go forward") and he himself visited Scully in a dream ("I have been on a bridge linking two worlds"). I don't know whether everyone in the US now speaks like that or not, but these phrases sound like direct lifts from recent Bill Clinton campaign speeches. Is someone in the Presidential team moonlighting? Perhaps it is the same person who suggested that Hillary seek advice from the dead Eleanor Roosevelt not long ago. If so, expect Scully and Mulder to be consulting George Washington soon.

As we know, such credulousness is not just an American trait. I can easily imagine an indigenous version - The Y Files, with Spooky Vorderman as Agent Silly and Chris Choi as Moulder - in which the Government keeps suppressing the truth about pub poltergeists, dowsing and telepathic budgies. But for the time being we must make do with a much cheaper and more fundamentally British mystery series, Neverwhere (BBC2), which was screened at the same time on Thursday as The X Files, and whose debt to the Dr Who tradition was explicitly acknowledged by its "devisers", one of whom is none other than Lenny Henry.

Let me attempt a synopsis: young Scot in London helps mystery woman, who is being chased by demonic baddies. She is royalty from a world beneath the streets of London. Young Scot (like Orpheus) follows her into the underworld and is seized; end of episode.

But the plot is not what is important - it is nothing more than a skimpy washing line. No, what I most love is the colourful washing itself: the sets, the costumes and the names. Let's take the locations first. An awful lot of scampering and speedy ladder-ascending/ descending gets done underground, suggesting that both designers and actors were trained on The Crystal Maze. The latter rush along old bits of blue-lit sewer, drop down manholes, crawl through red tunnels and stand Harry Lime-like in dark corners, to be picked out by the baleful light of a halogen tube. Odd alleys in London come in handy, too, once the props department has covered them in designer cardboard and carefully selected mounds of theatrical rubbish ("Shouldn't we add one of those hamburger wrappers for verisimilitude, Quentin?").

The costumes are wonderful. The slinky black goodie (Lenny Henry's alter ego?) has short blond hair with rasta extensions, a long, heavy leather overcoat and tight, easy-split Darcy trousers. No wonder he scales all the walls and ladders rather gingerly. For some reason the baddies wear white Pacamacs and are - with one exception - fat. They also bite the heads off live rats, so they aren't all bad.

It is the women, however, who best exemplify the long British TV and film tradition of fantasy costume design. The principal heroine, in roughed- up state, has a smudge of dirt on either cheek, back-combed hair, a pert nose, is wrapped up in an old rug and is responding to an instruction which reads: "She stares up with frightened eyes." When cleaned up and ready for action she sports a slinky black number with laced bodice, tight leggings, silvery dagger and sudden propensity for snappy one-liners. It has been thus ever since Dr Who began, was thus in movies such as One Million Years BC, and will remain thus until women take over the scripting and directing of sci-fi and fantasy series.

Finally, on to the characters themselves. Disillusioned, naive Scots yuppies are very hip, so I won't go on about the hero. Princess Door is a Princess Leia stereotype, who talks to rats and pigeons. She speaks in English and they apparently reply in warbles and squeaks. In an American series it would be the other way around, leading to a credit for an expensive "rat language coach". It is much cheaper just to let the animal make its noises and nod your head sagely as if in agreement.

Old tight-trousers is a Pimpernel with attitude named the Marquis of Carabas - which (as every skoolboy kno) also happens to be the name that Puss in Boots bestowed upon his low-born master, when trying to pass him off as a member of the French aristocracy. The Pacamacs are bloodless psychopaths with a line in enormous knuckledusters, disgusting eating and expectorating habits, and convoluted mock-literary language. There is Old Bailey, a pigeon fancier who lives on top of St Pancras station in a feather wigwam and eats rooks; and there are the motley denizens of Neverwhere themselves - rough-sleepers above ground, Iliaster and Lord Rat-Speaker down below.

Neverwhere is, of course, a Neverland under our feet. The lost boys are the homeless and the outcast. Door is Tinkerbell, the Pacamacs are Captain Hooks. It is also literally Underworld (or, I suppose, Underwhere): a fantasy for the Newbury age, with grunge characters and crusties pursued by the sci-fi equivalent of private security guards, who are determined to put an end to such essentially harmless alternative lifestyles as talking to vermin and living in total squalor.

Weirdness and the occult are not restricted to a particular drama genre, however. Nigel Le Vaillant, the lead actor in Dangerfield (BBC1, Friday), does not talk much to animals (in fact he doesn't talk much to anybody. He lets his jaw do all the talking. And it says, "I am a hunk. And I am a doctor. Would you like to swoon now?"). But he did this week get to exchange premonitions with a medium. I am praying that she turns out be a fraud in the next episode, and that all this fashion for spookiness does not spill over into the GP/police-doctor/ vet/pathologist/police-vet genre, where science and tea have traditionally ruled.

Anyway, I am not interested in the mystery of the blue rope and the old murders, thank you very much. Dr Dangerfield has a new colleague, a middle- aged and highly attractive Irish woman doctor, Dr Robbins ("call me Annie"), who looks like a cross between Mary Robinson (President of the Irish Republic, for those ignoramuses who are on a break from playing Dungeons and Dragons) and Helena Kennedy. She's happily married to a tolerant husband - a New Man to boot - and is thus just asking to have her life torn apart by a passionate affair with a lantern-jawed and monosyllabic hunk. One just knows that sex with Dangerfield won't be much fun. There'll be no tickling or inane giggles, or chasing round the bedroom, or swapping of undergarments, or sudden applications of creme fraiche to each other's neverwheres. It'll all be mouth-crushing, "We can't do this", needful grunting and tears at bedtime stuff.

What might be fun would be to swap the characters from Neverwhere with those from Dangerfield (Dangerwhere? Neverfield?), and have a black doctor in dreadlocks and pantaloons swan around Warwickshire, while a middle-aged Irish President with smudges gets to have dinner with a nest of rats. Not possible? Ah, Horatio. There are more things in heaven and earth ...