You'd better hum the theme to Bonanza to get in the mood for this one.

That won't provide the foggiest clue to the post-gothic music of English quintet Fields of the Nephilim, but you'll get an idea of what they look like.

Fog itself, in fact, is the real clue to this ponderous rock group, which follows up its newly released North American debut album, The Nephilim, with an appearance Tuesday night at the Siboney. By all accounts, the Neffs, as the U.K. press fondly calls them, shroud themselves in great, wafting gobs of dry-ice clouds on stage. You might not actually see them until halfway through the show.

And two things are possible for those who survive that far into the show: Some may well assure themselves that indeed, "It came from England, and if we ask nicely, maybe it'll go home." Others, mind you, could well be swept up into the transient ranks of the Bonanzas.

Yes, Bonanzas. Guitarist Peter Yates explains: "I guess our Bonanzas are much like the Deadhead phenomenon that surrounds the Grateful Dead - really hard-core fans who actually follow us along from show to show.

"We just finished a five-week tour of Europe, and about 60 of them were there at every gig. They call themselves Bonanzas after the TV show, because that's how we dress. It's a tongue-in-cheek thing, of course."

Yates, speaking from Los Angeles, is weary from the city-hopping tour designed to promote the new album, a condensed tracing of highlights from the 5-year-old group's two British releases. After all, trying to describe the Neffs music isn't easy.

"I can't even begin," he signs. "One thing, though. You can't just listen to this record once and cast it aside. It has to be heard several times to be understood, and even then, it makes much more sense once you've seen us live."

A combination of brooding guitar bombast and grave, cryptic ululations from vocalist/lyricist Carl McCoy (one U.K. paper says he has one "coffin lid" of a voice), the Neffs' sound trades in mystical, swirling rhythms atypical of the current British scene. Flavor of the week, they are not.

"We get slagged off as goths by some writers, but gothic rock to me is something like Alien Sex Fiend," says Yates. "I don't think what we do is limited to that small a category."

The name Nephilim, which comes from an ancient Hebrew word for a mythical race of giants, may conjure shades of Spinal Tap, but at least the group is willing to view itself with some degree of humor.

Earlier this year, for example, all five members were dragged out of their hotel in the middle of the night by a police drug squad in Nottingham, England. As part of the stage show, the members literally splatter themselves with flour, to accent their other-worldly presence. The police were tipped off that the those powder-encrusted T-shirts might point to something illicit.