“The Simpsons” has been on the air longer than “Gunsmoke,” but that’s not a reason to watch the show’s 20th-anniversary special on Fox this Sunday night. Tributes to even the best shows can smother them in pomposity. “The Simpsons 20th Anniversary Special: In 3-D! On Ice!” is worth a look because  as the self-mocking title suggests  it doesn’t make too big a deal of one of the most innovative, influential and irresistibly irreverent shows in the history of television.

The event opens with a new episode, “Once Upon a Time in Springfield,” followed by an hourlong documentary by Morgan Spurlock, the director and star of “Super Size Me.” The writers make a few smirking references to the show’s longevity  Bart at the blackboard writes over and over, “The world may end in 2012, but this show won’t.” Mostly it’s a typical half-hour of animated lunacy and a reminder to apostates of why they loved the show in the first place.

And that dose of unadulterated “Simpsons” is helpful because it’s so hard for even a well-made documentary to convey how deftly the writers blend highbrow humor into slapstick cartoon antics, not to mention social satire, irony, parody, self-mockery, political commentary and an unceasing sendup of television, particularly its home network.

Earlier animated series, notably “The Rocky and Bullwinkle Show” and “The Flintstones,” worked on two levels, but “The Simpsons” works on one more: children, adults, and adults who read a lot. And that sophistication, lightly worn, has schooled viewers over two decades. Not every show slips in references to “The Music Man” or Edgar Allan Poe, but those that do find an audience for it, thanks in large part to “The Simpsons.” (It’s not just “South Park” or “Family Guy.” Even “Gossip Girl” has sardonic allusions to “Breakfast at Tiffany’s” and Eliot Spitzer.)