"Is the pig ready?" someone calls out, and a genial and balding chap named Frank Oz climbs into a hole under a stage. There is a pig on his arm: the tempestuous Miss Piggy, no less. Either daringly or foolhardily, "Of Muppets and Men," at 5 today on Channel 20, reveals the world behind and beneath the scenes of the biggest TV hit in the world, "The Muppet Show."

Strangely enough, the program -- essentially a one-hour promo by and for Jim Henson's Muppets -- can jump from a scene in which a Muppet is clearly no more than a lifeless appendage on a human arm to a clip from "The Muppet Show" in which the Muppet becomes real again, and the fantasy holds up. Perhaps it has something to do with the essential integrity of the illusion, or the masterful way it is sustained by Henson and his talented staff, who on this show finally get to appear as themselves and not just a bunch of talented hands.

Knowing and seeing "how it's done" shouldn't spoil the fun for anyone but the most insecure. The show beneath the show is almost as entertaining anyway -- a squadron of puppeteers with German-waiter Muppets on the their arms sing and march about while young assistants dump spaghetti on the waiters' plates from the wings; the crew members huddle together with their arms sticking up through a model of a Viking ship on which Muppets are singing "In the Navy."

It's all so clever, and such pure television, and so engagingly imaginative compared with most of the human shows on the air; it's more human, too. This program, directed by Harley Cokliss and Peter Berry and made available to stations now airing syndicated "Muppet Show" reruns, visits not only the London studio where the show is taped but also the workshop where Muppets are constructed and where sits a tray full of plastic eyes.

Oz affectionately refers to the Muppets as "these idiots" and marvels at their love for hokey material; "The Muppet Show" was a way to recycle years and years of accumulated shtick that most of the Muppet creators probably first saw on television as kids. Henson says sustaining the illusion of Muppet mobility is "kind of a game we play with the audience," such a friendly game that everyone's in on the joke when Kermit does a tap-dancing number in which his feet are never shown.

A guest star montage or two indicates the appeal this game has had for a wide range of celebrities: Zero Mostel, Peter Sellers, Beverly Sills, Milton Berle, Liza Minnelli, Linda Ronstadt, Glenda Jackson and Rudolf Nureyev. They let themselves be put through the paces of what writer Jerry Juhl calls "almost any insane fantasy you can think of."

Henson's is the voice heard most often in the running commentary; he seems generous enough in spreading praise around to his staff (though a more accurate title for the program would be "Of Muppets and Men and Women"). Henson is a bland fellow, a trifle on the dull side, but put a frog on his arm and he's gang-busters. "This is a nice group of people, and we have a lot of fun," he says innocuously. Somehow it isn't at all hard to believe.