It's life imitating art imitating life imitating art . . . ad nauseam.

Consider: The 1984 film "This Is Spinal Tap" introduced us to a gracelessly aging British heavy metal band -- with a propensity ,, for hiring drummers who die by spontaneous combustion -- that disintegrated before our eyes. It was purely and clearly fictional, a wickedly funny and admirably accurate music business lampoon penned by director Rob Reiner and actors Christopher Guest, Michael McKean and Harry Shearer.

Eight years later, the band Spinal Tap has re-emerged and seems to be playing for keeps. On March 17, the group will release a new album, titled "Break Like the Wind," which features 13 original songs and guest appearances by Cher, Jeff Beck, Joe Satriani, Dweezil Zappa and Guns N' Roses' guitarist Slash. That week, Billboard magazine will publish a commemorative special section celebrating the band's 25th anniversary. A world tour is planned, as well as a summer TV special.

Somehow, the line between satire and reality has grown as fuzzy the hair on Weird Al Yankovic's head.

"It still is a parody," says Mr. McKean, the alter ego of Tap front man David St. Hubbins. "But we had no intention of doing a sequel [to the film]. So we're doing something that kind of invites the audience in a little more. And the audience we seem to attract is playing the part of Spinal Tap's fans just like we're playing Spinal Tap."

This is where things get cloudy. McKean, Guest and Shearer -- who collectively boast musical credits that include National Lampoon revues and backing Arlo Guthrie -- talk of the characters and the band in the third person. "In the movie, it was very clearly an alternate universe," says Shearer, 48, who doubles as Tap bassist Derek Smalls.

Now, it's not so clear. In the same breath, Shearer likes to make it clear that Spinal Tap is "a real band. They do exist. They do play. In the movie, all the references were to people who were fictional; now, Spinal Tap has to interact with the real world, with real people. On this album, it's the real Jeff Beck, the real Cher. Now we're not making fun of it. This is the thing itself and, yeah, those lines do blur. "

Adds 44-year-old Guest, whose Nigel Tufnel looks like Beck's lost brother: "It's certainly surreal, because for years you've read things about this fictional band. Of course, now it's not a fictional band."

Spinal Tap's journey from screen to stage is the result of an unexpected cult following that grew from the film. The group was conceived in 1979 for a skit on an obscure television special called "The TV Show." They enjoyed the Tap bit so much that they began exploring ways to showcase the band.

For anyone with a modicum of musical knowledge -- whether they've actually performed or simply spent a few minutes backstage -- "This Is Spinal Tap" was a largely ad-libbed hoot that dug beyond heavy metal stereotypes to poke poignant, pointed fun at the music industry and what Shearer calls the kind of "nameless, faceless, mediocre bands that for no good reason survive a million years."

And in Spinal Tap's case, we're talking about guys who write songs touting the virtues of the gluteus maximus ("Big Bottom") and call it art.

The film poses as a documentary in which director and big Tap Marti DiBergi (Reiner) goes on the road with the band. He captures a variety of hysterically funny mishaps -- stage props that don't work, mistaken bookings at an Air Force base and an amusement park, a record store autograph session for which nobody shows up -- and stages interviews in which the Tap members speak earnestly about abilities, achievements and pretensions.

The magic of "This Is Spinal Tap" is in its understated, poker-faced tone. The blatant slapstick is held to a minimum, and even the music is passable metal fare. "There's nothing on the surface that's necessarily funny about this stuff," says Guest. "We play the way Spinal Tap would play. We have a very good idea of who the characters we're playing are and what kind of songs they would write and how they would play them."

Many of the incidents are based on real stories -- the military booking, for instance, actually happened to the British band Uriah Heep. And there were equally authentic inspirations for the characters, including a pompous record company president, a grating label spokeswoman, a schnorry record promotion man and the musicians themselves.

"I swear to God that when I watched that movie, I related to so much of that stuff for real," veteran heavy metal singer Ozzy Osbourne says of the film. "I know it was a comedy film, but it may as well have been a documentary. Spinal Tap reminded me so much of what we were like in Black Sabbath. Everyone sitting around me was laughing, and I was saying, 'What are you laughing about? That really happened.' "

Some viewers figured it must have happened -- even if they had never heard of Spinal Tap. Guest, McKean and Shearer still laugh about screening audiences' response cards that made it clear not everyone was getting the joke. "A lot of people asked us, 'Why didn't you make a movie about a better band?' " Shearer says. And record store owners marveled at the requests they got for the mock albums advertised in the film's soundtrack.

Shearer, Guest and McKean -- who can quickly and smoothly slip into character at the slightest provocation -- have worked to keep Spinal Tap in the public eye. Shearer and McKean sang on "Stars," a hard rock all-star benefit record. Guest, as Tufnel, posed for a popular Marshall amplifier poster. And the group has appeared on award shows and at music business trade conventions.

Toto guitarist Steve Lukather, who produced four tracks on "Break Like the Wind," says musicians love Spinal Tap because "it's a welcome breath of fresh air. It's a subtle cartooning of the medium without taking a huge [shot] on it. . . . And it's nice that these guys can really play. It's not bad music; in fact, I think it's as catchy if not more creative than a lot of the competition."

The fact that McKean, Guest and Shearer are musically adroit is what makes it most difficult to gauge the veracity of the current Tap activity. Though it takes some broad swipes at the genre's stereotypes -- misogynistic lyrics, bombastic music -- "Break Like the Wind" can be played credibly next to most hard rock albums. And there are moments where it actually rises to metallic heights, such as Cher's duet with McKean on "Begin Again" and the frantic guitar soloing of Beck, Slash, Satriani and Lukather on the title track.

"We have taken the musical part of this very seriously," says Guest. "When Cher sang on the record, when she heard the track she was going to sing on, she asked, 'Who's playing this?' and we said 'Well, we are,' and she was shocked."