Much like the beloved television special, the story is a series of vignettes with an underlying plot. Unlike the beloved television special, every moment forces you to comfort yourself with the fact that existence is ultimately meaningless and the terrible knowledge you've gained will one day be lost to the merciful void. Someone wrote about Lucy having sex with Snoopy, and only oblivion can erase that from your mind.
The dog eyed the crotch in front of him, dropped the blanket and sniffed. 'Whoa!' he thought, 'pussy smell.' He stuck his wet nose deeper into her crotch, poking into the little slit beneath the panties.
It gets much more explicit than that, but I don't want a lynch mob outside my house, so let's skip ahead. Linus and Charlie Brown go Christmas-tree shopping, during which Linus talks about how he blackmailed Peppermint Patty and Marcie into filthy sex. Then he teaches Charlie Brown about the joys of ass play, as one does after a successful tree purchase.
Even the tree is ashamed.
Linus put his hand between Charlie Brown's legs and fingered his little brown ass pucker. Charlie Brown had never thought about his asshole as a sexual area, but Linus's finger there felt great.
Then comes the big Christmas party, which soon devolves into an uncomfortable Peanuts orgy. Charlie Brown gets to have nasty sex with the little red-headed girl, which is hardly canonical, and Lucy embraces her perversions and fucks a dog in public. But Woodstock and his other bird friends don't get involved, because that would be going too far.