New network, same old Buffy.

The switch from The WB to UPN hasn't fazed "Buffy the Vampire Slayer" one bit. The show is still the most ambitious, inventive and well-written on television. (What? You think that honor goes to the ever-pretentious "West Wing?" Yeah, you probably thought "Sports Night" was funny, too.)

The transition also has worked wonders for UPN, giving that network, best known for "WWF Smackdown," a touch of class and a leg up on The WB, which is left with "Smallville" — "Superman" meets "Dawson's Creek" — and not much else.

Last week's "Buffy" installment is a perfect example.

"Buffy" (Tuesdays at 7 p.m., UPN 15) is by no means the first TV series to have a musical episode. It isn't even the first genre series to do so. ("Xena: Warrior Princess" turned in a credible effort, and even the Sci-Fi Channel series "LEXX" has tried it, not that I'd recommend watching "LEXX" under any circumstances.) But, as usual, "Buffy" just does it better than everyone else.

Most of the credit goes to the series' creator and executive producer, Joss Whedon, who turned the show over to others while spending months on his musical episode, "Once More, With Feeling." And his gamble paid off. While Whedon's scriptwriting talents were missed, he more than makes up for his absence.

"Once More, With Feeling" mixes musical numbers inspired by everything from Rodgers and Hammerstein to Andrew Lloyd Weber. And just to prove that the episode is no mere gimmick, Whedon makes sure the songs advance all of his dangling plot threads, some of which have been hanging about for more than a year.

Buffy (Sarah Michelle Gellar) finally tells her friends why she isn't happy they resurrected her from the dead. Former bad boy Spike (James Marsters) finally tells Buffy he is in love with her. Buffy's mentor, Giles (Anthony Stewart Head), finally reveals he is leaving. Xander and Anya (Nicholas Brendan and Emma Caulfield) finally confess their pre-wedding jitters to each other. And Tara (Amber Benson) finally discovers the true extent to which her girlfriend, Willow (Alyson Hannigan), has been misusing magic.

And, of course, all this happens in song.

Whedon knows his cast's strengths and weaknesses. Head and Benson are by far the best singers, so he gives them the most challenging numbers, including an overlapping duet.

Spike has been a Billy Idol clone since day one, so Whedon gives him a rousing rock ballad. (Sometimes you just gotta go with the obvious.)

Brendan isn't much of a singer, so Whedon wisely pairs him with Caulfield for a semi-talky Fred-and-Ginger-style dance sequence. (Caul-field can sing, so Anya also gets a hilarious solo about evil bunny rabbits. You just have to see it.)

Speaking of dancing, that is obviously Michelle Trachtenberg's strong suit. So her character, Dawn, gets a slinky dance number with the episode's villain, a musical demon who is the reason why everyone is spontaneously breaking into song.

That leaves only Willow and Buffy.

Poor Alyson may be the cutest cast member — and probably the best actress — but she must not be able to sing a note; Whedon gives her only one solo line, and it sort of creaks out of her throat like a death rattle. (Oh well. We still love you, Alyson.)

But none of this would work without Gellar.

Her voice isn't quite strong enough to carry the power ballad that sets up the finale, but she turns in a better performance than probably even her most loyal fans could have expected.

Along with "Hush" — another "gimmick" episode, this time without dialogue — this is "Buffy" at its best.

Usually after six years, a TV series becomes a shallow parody of its former self. Look at what happened to "The X-Files." (Or better yet, don't.) But "Buffy" remains fresh. And that gives us hope that it can weather the changes ahead, such as Head's impending departure.

So, if you missed the musical episode the first time around, be sure to catch the repeat, which airs Friday night at 7.