Sarah Chalke and Zach Braff do the Hippocratic shimmy on the tune-filled ’Scrubs.’

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"Scrubs: My Musical." Tomorrow night at 9, NBC.

Musical episodes of nonmusical TV shows are rare, but they happen - and even more rarely, some happen to be great.

Tomorrow night’s installment of NBC’s "Scrubs," subtitled "My Musical" (at 9), earns that praise.

Not all musical TV episodes are created equal. "The Drew Carey Show" and "Ally McBeal" did them, entertainingly, using largely recycled songs (though James Naughton as Ally’s dad, playing the piano and singing Randy Newman’s "Real Emotional Girl" to his TV daughter, was a perfect moment).

The series "7th Heaven" did a musical episode that didn’t work, and "The Simpsons" regularly does ones that do, like its seminal musical version of "A Streetcar Named Desire."

"St. Elsewhere" did an episode built around music videos, and even "Oz," bizarrely, mounted a musical once. "Xena: Warrior Princess" did two of them, and the first, "The Bitter Suite," was excellent - but in terms of delivering a special musical episode, nothing matches the excellence of Joss Whedon’s "Once More With Feeling" musical episode of "Buffy the Vampire Slayer."

Tomorrow night, though, "Scrubs" comes impressively close.

Where Whedon wrote the music and lyrics himself, the "Scrubs" folks - series creator Bill Lawrence, writer Debra Fordham and director Will Mackenzie - enlisted ringers for help. Specifically, they enlisted Stephanie D’Abruzzo, the original female lead (and dual-personality puppeteer) of "Avenue Q," as guest star, playing a patient who thinks everyone around her is communicating in song.

She arrives with ringers off-camera as well. "Avenue Q" composers Robert Lopez and Jeff Marx, along with Paul Perry, work with Fordham on the lyrics, and Lopez, Marx, Perry and Jan Stevens team up for the music. Ten songs are crammed into 22 minutes of "Scrubs" screen time, including two songs that sound only an avenue or two removed from "Q."

Zach Braff and Donald Faison, as best-buddy doctors J.D. and Turk, sing them both: "Everything Comes Down to Poo" and "Guy Love." As for the vocal standouts in the cast, regular "Scrubs" watchers already know about Sam Lloyd as Ted (and his recurring appearances with his a cappella quartet), but Judy Reyes, as Carla the nurse, bursts out as a genuine musical talent. Her song with Turk, "For the Last Time I’m Dominican," is delightful - and finds some rhymes for Dominican, including "inner man," that are funny just because they stretch but don’t snap.

There’s a "Grease" parody, and a song that requires John C. McGinley’s angry Dr. Cox to sing as quickly as he talks, and other treats, many of which reward long-time "Scrubs" fans with in-joke references (when Neil Flynn, as the janitor, explains his hatred of J.D. by singing "It all started with a penny in the door," it’s extra funny if you remember that it did, in the show’s very first episode).

As always with "Scrubs," though, there are serious notes to be struck - and when D’Abruzzo, as Mrs. Miller, gets a diagnosis that is imminently life-threatening, she has to face the music, and risk learning whether she can live without it.

If I could carry a tune, I’d be singing the praises of this "Scrubs" episode - but you’re better off with me just typing them.