The idea, when they hit upon it, was irresistible: Make a fake documentary about a fake rock band, sketching out the plot in advance but improvising the dialogue to make it feel spontaneous. The group, they decided, would be called Spinal Tap, a name meant to parody the gothic pretensions of heavy metal. They—“they” being Harry Shearer, Christopher Guest, Michael McKean, and Rob Reiner, four comedic actors in their 30s—spent hours inventing the band's extensive backstory, which they would memorize before cameras rolled.

Shearer, who plays bassist Derek Smalls, says everything from the title of the band's first single (“Gimme Some Money”) to the cause of death of one of their many drummers (spontaneous combustion) came out of these early meetings, where the goal was to crack one another up. “That's the scientific term for our methods: sitting around laughing,” he says. A beat, and then he adds with a wry smile: “So why should we be paid for it?”

In the 33 years since the release of This Is Spinal Tap—which Guest once labeled “spomage,” for its rare mixture of spoof and homage—the film has become not just a beloved cult classic but also a national treasure. The Library of Congress has deemed it “culturally, historically, or aesthetically significant.” A whole generation of comedians (and not just comedians) has grown up quoting it. Ricky Gervais called it “a huge influence” and “my favorite comedy of all time.” But even as the film's most hilarious moments have become touchstones—guitarist Nigel Tufnel's explanation, say, of precisely why an amp that goes to 11 is louder than one that goes to 10 (“Well, it's one louder, isn't it?”)—the royalty checks from This Is Spinal Tap have remained suspiciously tiny. What started as a send-up of the music business has turned into a lesson in the absurdities of the film business.

“I think we all shared this inchoate fantasy that because we'd made fun of this stuff, it wouldn't happen to us,” Shearer tells me not long after I meet him at his home in New Orleans. He's talking about the real-life indignities that the fake band has endured over the years, but there's a hint of the creators' current predicament in there as well: “When we started getting fucked over, we were just like, ‘My God!’—in blank disbelief.”

The guys are all believers now, which is why they've filed a $400 million lawsuit against the French conglomerate Vivendi and its film subsidiary Studiocanal, which owns the rights to Spinal Tap. Among the suit's charges: fraud, breach of contract, and abuse of power “as part of an intentional scheme” to deprive the creators of what is legitimately theirs. (Vivendi's lawyers deny any wrongdoing, calling the $400 million figure “fanciful” at best.)

Central to the band's case is the claim that Vivendi has been hiding millions of dollars in profits from Spinal Tap by packaging the movie with flops, an accounting trick known as cross-collateralizing. The film, the company insists, is barely breaking even. “Do they believe their own financial statements?” Shearer asks. “Because if they're believing what they send to us, I think they need medical attention.”

Shearer filed the initial complaint in October; Guest, McKean, and Reiner joined as co-plaintiffs in February. (They declined to be interviewed for this story, sending word that Shearer would speak for the lawsuit and the band.) In a February motion to dismiss the case, Vivendi's lawyers claim Spinal Tap, which Shearer says cost just $2.25 million to make, has generated less than $5 million in U.S. ticket sales. But box-office receipts are only a portion of what Shearer and the others are after. Their 1982 contract says they are due 40 percent of all net receipts that emanate from the film and 50 percent of gross receipts related to the music. Since the film was released, there have been numerous VHS and DVD editions, follow-up albums, and reunion concerts, and all sorts of merchandise sold along the way. To quote the movie, Spinal Tap has become, like a cherished electric guitar, “famous for its sustain.” Yet, according to the band's filing, the four creators' supposed share of total worldwide merchandising income between 1984 and 2013 was only $81. Their share of music sales for a similar period was $98.

“If you don't fight, you're really just roadkill,” Shearer says, his wiry five-foot-eight-inch frame straining over his dining-room table. “If I had a slogan, that would be it: Fight or be roadkill.” The credo explains not only this lawsuit but also Shearer's approach to nearly everything. “I've, through no fault of my own, gained a certain reputation in the business as a troublemaker,” he says. Most famously, he battled 20th Century Fox—and won—over the terms of his contract on The Simpsons. (He voices 23 characters on the show, including Ned Flanders, Mr. Burns, and Principal Skinner.) When I tell Shearer that I've read that he considers himself an Old Testament guy, he grins. “That's true. My God is a God of vengeance.”