The cast has met at one member's house for a long study night, sometime around Easter. After a few hours of this, eventually they all start nodding off. They find themselves meeting each other in the same dreamscape, except for one missing character (Jack Pennington, the hero I normally played). After encountering the White Rabbit-slash-Easter Bunny*, they eventually find Jack, dressed as the Red King, asleep under a tree. A pair of NPCs (some variant of Tweedledee and Tweedledum, as in the original story) warns them not to wake up the King. They're part of his dream after all, and might wink out if it ends! The heroes learn about a possible means of returning to the waking world: hunt down and capture a Snark, which lives somewhere in this tulgey wood.
It turns out that Snarks look much like maniacal rabbits, or possibly jackalopes--at least part of the time. These shapeshifting creatures should prove elusive for some time while the cast tries out more and more outrageous ideas for catching one.
Eventually, the most dangerous Snark of all arrives on the scene: the Jabberwock. John Tenniel's illustration in Through the Looking-glass shows this beast with large chisel teeth and protruding eyes like a rabbit--which inspired me to push the bunny theme to absurd extremes. Assuming the cast managed to acquire a vorpal sword during the adventure, then a cast member (most likely the resident Slayer) can use it to slay the Jabberwock. This monster is big and nasty, and worst of all, it's a Boojum:
"'But oh, beamish nephew, beware of the day,
If your Snark be a Boojum! For then
You will softly and suddenly vanish away,
And never be met with again!'
Let the Boojum slowly "vanish" cast members as the fight progresses, making the situation more and more dire, but leave them some hope--Hope is one of the most effective weapons against Snarks, after all. This is where the White Hats can do their thing to encourage the Slayer, keep the Jabberwock distracted and confused, and so on.
Eventually, the surviving heroes triumph--and wake up, because it was all a dream. Cast members who were "vanished" are still there, but need some poking and prodding before they'll wake up. Jack can't be awakened until everyone else has--he was the Boojum's first victim, and is left with a severe bunny phobia.
My working title, "Leporiphobia," means fear of rabbits, and is a reference to one of the ex-Vengeance Demon Anya's little quirks. If you adapt this idea to run with the original Buffy cast, then Anya should become the Red King character. The other cast members get to experience a taste of how she feels about those creepy, evil bunnies--which might make her fear of them seem much less silly by the time the episode ends!
* I envisioned this character to be Jeffrey Jones cast as a chaos demon, wearing pajamas, a bathrobe, and bunny slippers, carrying a basket of eggs and a pocket watch. The picture I drew when I originally conceived "Leporiphobia" can be found here. (After seeing Stardust, though, I'd probably use Mark Williams if I ran this now.) The chaos demon might be responsible for the dream--intentionally or accidentally--but is more or less harmless.