Joss Whedon spent his summer vacation concocting a very special "Buffy the Vampire Slayer" for UPN — writing the score for a full-blown musical episode, then returning to write and direct it.

As when "Ally McBeal" enlisted Randy Newman to write a musical finale two seasons ago, tonight's special "Buffy" (at 8o'clock) is fun to watch to learn which cast members have the best singing voices and how their numbers will be written and staged.

With this show, though, the original song lyrics are so introspective and instructive, we also learn a great deal more about these familiar characters, even as they learn more about one another in shocking detail.

The ambitious scope of tonight's show, given the title "Once More, With Feeling," is impressive. Even more impressive is that Whedon, when mapping out this seventh "Buffy" episode of the season and planning its emotional arcs and pivotal plot turns, had to know where his characters would be at that point.

So rather than standing alone as a special episode, this musical "Buffy" picks up right where last week's show left off.

Buffy (Sarah Michelle Gellar) is still reeling from being raised from the dead, while Willow (Alyson Hannigan) is growing more powerful and reckless with her witchly magic since casting the spell that raised her. Xander (Nicholas Brendon) and Anya (Emma Caulfield) are engaged but nervous. Giles (Anthony Stewart Head) feels out of place as the oldest, and Dawn (Michelle Trachtenberg) feels equally out of place as the youngest. And Tara (Amber Benson) loves Willow, and Spike (James Marsters) pines for Buffy — but every character, by the end of the hour, will reveal a secret or shift an allegiance.

The best singers in the mix are Head, Marsters, Caulfield and Benson; guest star Hinton Battle, from Broadway, plays the demon whose spell has caused everyone in Sunnydale to break into song. (This includes one woman, musically fighting a parking ticket, who's played by "Buffy" co-executive producer Marti Noxon, in a Hitchcockian singing cameo.)

Everyone performs gamely, though. Most numbers are delightful and contain enough surprises that to describe them would be dampening part of the joy.

Whedon, however, is out for more than simple fun.

When he first has Tara singing "I'm caught in your spell" to Willow, it's a lesbian love song. Later, when the reprise arrives, it's a realization of being manipulated magically by a loved one. In another song, "Walk Through the Fire," no fewer than seven different characters are heard from, singing both individually and as one.

Tonight's "Buffy," smart and surprising in so many ways, is no "Cop Rock" — and that's a compliment. Oh, and if you're setting your VCR to tape this episode, watch out: It runs 10 minutes longer, so adjust your timer and choose your tape or tape speed accordingly.