Life, every so often, really does turn out to be a Christopher Guest movie. Gigantic (A Tale of Two Johns) is an unintentionally funny fanzine-flavored documentary about They Might Be Giants, the indie-rock duo that has spent 20 years flitting along the invisible line between the cult and the mainstream. If you thought that the party-down metalheads in ''The Decline of Western Civilization II'' were full of themselves, just wait until you enter the echo chamber of whimsical egomania that is alt-rock attitude.
The bandmates, plumpish, bespectacled John Flansburgh (guitar) and lank-haired John Linnell (accordion and keyboards), are Ur-ironic geeks who springboarded off the faux-naïf ditty rock of Jonathan Richman to do some infectious ditties of their own (like the single ''Don't Let's Start''). That's neither a small accomplishment nor, perhaps, a major one, but if, like me, you don't happen to be an indie-rock insider, to watch ''Gigantic'' is to think these two were the boho messiahs of the post-punk age. It's as if a little band called R.E.M. had never existed.
One associate likens TMBG to Lennon and McCartney, and the film goes on to present every minor fluctuation in the duo's career, such as their pre-Internet promotional gimmick Dial-a-Song, as if it were a momentous event. Throughout, Flansburgh and Linnell display a coy self-admiration masquerading as self-deprecation, recalling that of author Dave Eggers, who appears in the film to admire them further. Flansburgh is described as the sort of great soul who, when there isn't enough Mountain Dew on the bus, will venture out himself in the middle of the night to get more Dew, man! No, Christopher Guest didn't make that one up: It's the fizz that bubbles around pop stars who are giants in their own minds.