Whatever else it is -- and Heaven knows there's a lot going on -- "Six Feet Under" has to be the most challenging weekly drama on television. There just aren't that many TV entertainments that mess around with the eternal puzzles of existence, mortality and humanity's place (if any) in the universe.

This is a reality show that's actually about reality -- and infinite variations thereof.

When the series returns for its third season tomorrow night at 9 on HBO, some of the details will have changed. Elder brother Nate Jr., one of those who inherited the family's funeral home when the patriarch died in the very first episode, has survived brain surgery. Or maybe he hasn't; that and the major step he's made in life may both be illusory.

Each episode begins with the death of Fisher & Sons' next customer, whoever in Los Angeles that might be, but the opening death this season is Nate's own. Seemingly, that is. Since Peter Krause, as Nate, is the virtual star of the ensemble show, it's probably not spilling any beans to reveal that near-death is as close as Nate gets, though for him, that's clearly close enough.

Then again, it is conceivable that writer-producer Alan Ball could have killed Nate off and still kept Krause prominent in the cast, because on this show members of what might be called the Dead Generation (or does that sound like Grateful Dead fans?) have been known to mingle with living people.

In the second episode, for example, the corpses of a businessman and the disgruntled employee who shot him during a killing spree, and then committed suicide, lie side by side in the embalming room. Suddenly one corpse pops up to complain to the other; the businessman naturally finds death to be quite the definitive inconvenience. Freddy Rodriguez as Rico, the funeral home's artistic embalmer, is cool with it; he's accustomed to chatty stiffs.

That's another change: Rico is now not just the best li'l ol' body man in L.A. but a partner in the firm; the sign outside says Fisher & Diaz. Slightly dizzied by the upward mobility, Rico is hypersensitive to anything that might be interpreted as a slight from the Fisher brothers, the younger of whom is David, who's gay and played by Michael C. Hall.

Since the first episode, David has been seeing and sometimes living with a burly and handsome African American named Keith (Mathew St. Patrick), who has always seemed like the ideal life partner whatever one's sexual stance; among other virtues, he's patient to an almost superhuman degree. Indeed, why Keith continues to put up with that neurotic fussbudget David remains a mystery. This season, the two men are seeing a relationship counselor (Arye Gross) to try to work out David's problems.

David overanalyzes every moment of his life. When he and Keith are preparing dinner and Keith stops him from putting too much pepper into the pot, David announces ceremonially, "Okay -- I feel shamed." Later though, David -- auditioning for the local gay men's chorus -- contributes mightily to the most memorable part of the episode, a montage set to the beautiful song "Some Other Time" from the musical "On the Town."

The way Ball maneuvers his characters on the chessboard of life -- Frances Conroy as widow Ruth Fisher, Lauren Ambrose as her free-spirited (yet still somewhat repressed) daughter -- yields many a provocative surprise and few of the predictable cliches common to other dramas.

Kathy Bates joins the cast in the second episode as a friend helping Ruth's sister overcome an addiction to painkillers, and the terrific Catherine O'Hara, as a capriciously demanding actress who employs Nate's dumpy wife as a personal assistant, seems, as usual, permanently aglow, even though her character is a huge pain in the, uh, neck.

Two of the better actors in the show, however -- Rachel Griffiths as promiscuous Brenda and puckish Jeremy Sisto as her intriguingly insane brother -- don't show up in the first two episodes, which may mean they've been dispatched to the TV equivalent of the hereafter. They live on in memory and in the purgatory of reruns.

The opening scenes of the first new episode are quite unlike anything normally seen in episodic dramas, as Nate wanders through a disorienting netherworld apparently induced by the anesthesia keeping him asleep during surgery. At moments, the sequence recalls Keir Dullea's magical mystery tour at the end of "2001: A Space Odyssey," but without the light show.

It's an opening that is virtually guaranteed to seduce fans of the show into settling in for what promises to be another fascinating, unpredictable and occasionally confounding season. Yes, the return of "Six Feet Under" makes you want to dive right in again.

Nate (Peter Krause) experiences the family business from the other side.Frances Conroy as funeral home matriarch Ruth Fisher in HBO's always-surprising "Six Feet Under."