I've watched The Simpsons with delight, and with rather dedicated regularity, ever since they first popped up on Fox's The Tracey Ullman Show 23 years ago. And though I'm merely noting, not complaining, last Sunday's Christmas special ended with what I consider the naughtiest joke in the lengthy history of Matt Groening's cartoon clan. And yes, it involved the week's special "human" guest star, Katy Perry...

The final segment of Sunday's Christmas special reimagined The Simpsons as Muppet-like puppets -- a knowing nod to pop singer Katy Perry's recent visit to Sesame Street, a musical duet with Elmo that was cut by an absurdly timid PBS because Perry's outfit and demeanor were deemed a bit too risque for a children's show.

So The Simpsons, of course, played with and attacked that premise by inviting Perry to appear, and pushing the boundaries way past the neighborhood boundaries of Sesame Street. This had to be seen to be believed -- so, today, I hereby present a sort of slideshow, with annotated images from Sunday's final segment.

We start, in the photo at the top of the column, with Perry making an unannounced stop at the Simpsons' house. She's decked out in a form-fitting red mini-dress emblazoned with the faces of Simpsons characters, and there to visit her boyfriend... Moe.

"That's right," Moe says defiantly, and not a little proudly. "She's into puppets!"

The irascible Mr. Burns begins to complain, but Katy Perry quiets him by flashing a smile and reaching out to embrace him.

"Someone totally needs a hug," she says.

Immediately, Mr. Burns changes his tune -- and alludes to one of hers.

"I kissed a girl," he says, "and I liked it."

Watching all this, from the balcony, are Grandpa Simpson and his aging buddy, acting as a yellow-skinned version of Statler and Waldorf from The Muppet Show.

"Well, it's been a long run," Grandpa mutters, "but I think this will kill it."

"What?" he's asked. "The Simpsons?"

"No," Grandpa replies. "Christmas."

Meanwhile, Mr. Burns demands that the Simpsons, Moe and Katy Perry all sing a Christmas carol -- what he calls "the unabridged '39 Days of Christmas.'"

And they do - with everyone, Katy Perry included, enumerating such previously unknown holiday delights as "twenty-six anarchists bombing" and, my favorite, "eighteen gungas dinning."

Then comes the envelope-pushing final joke.

Over the closing credits, we cut to a shot of Moe the puppet, trying in vain to hop high enough to kiss Katy Perry. Given the limitations of his puppet form, of course, he can't, so he settles for a hug and a smooch where he CAN reach.

"I'll just kiss your bellybutton," he tells her.

After a second, Katy's eyes get cartoonishly wide, and she says, "Oh. That's not my bellybutton..."

Then she flashes a smile. The scene cuts instantly, going to the familiar production-company-logo shot of a darkened movie theater. But it's not through yet, and neither is Katy Perry.

Though she's no longer seen, Katy Perry can still be heard, and she adds:

"...But I didn't say stop."