There’s a full moon above the mortuary

After a mysterious two-week disappearance, the body of Lisa (Lili Taylor), Nate Fisher’s (Peter Krause) sweet, gentle, murderously passive-aggressive wife, washed ashore somewhere near Santa Barbara.

The news, which came at the end of the final episode of “Six Feet Under” last June, broke the fever that had raged throughout much the third season. Thanks in part to Taylor’s moony presence, the congenitally repressed Fisher family had plunged headlong into lunacy.

Season four begins as if emerging from underwater, a gloomy lugubriousness replacing last season’s pitched desperation. It’s a logical respite, but too many threads were left loose for the Fishers’ lives not to start unraveling.

Claire (Lauren Ambrose) found out she was pregnant by her bisexual boyfriend, Russell (Ben Foster), and secretly had an abortion; David (Michael C. Hall) decided to end his tortured relationship with the angry, self-righteous Keith (Mathew St. Patrick) and moved out; Rico (Freddy Rodriguez) brought his wife, Vanessa (Justina Machado), back from the edge of self-destruction; and Ruth (Frances Conroy) impulsively married the vaguely unsettling George Sibley (James Cromwell), a man about whom she knew next to nothing -- except that he had been married six times before.

Nate, already so consumed with guilt that he seemed constantly on the verge of rending his garments, flew into a masochistic rage at the news of Lisa’s death. He showed up bleeding at the door of his ex-girlfriend, recovering sexaholic Brenda (Rachel Griffiths).

Sunday’s episode, titled “Falling Into Place,” does not reveal many details of Lisa’s death. We learn that she drowned, though not how or why. But one thing about the way she dies is suitably symbolic of the family’s disintegration: Apparently a drowned body eventually explodes from the pressure of expanding gases.

“Six Feet Under” always has been cast in a sort of lunar glow, and this seems truer than ever at the start of this season. New characters and story lines are introduced a few episodes in: Claire makes a couple of female friends (one of them played by Mena Suvari); Ruth discovers, in the most unpleasant way imaginable, that George has a son she didn’t know about (and damages one of her own relationships in the process); Brenda discovers something unexpected about the sex life of her new boyfriend, Joe (Justin Theroux); and Keith takes a new job as security guard to the stars.

As always, there’s something about the way the lovers pull toward and push away from each other, as if governed by some unseen tide, that is the main attraction of “Six Feet Under.” David takes Keith back on the flimsiest of pretexts.

Despite her burgeoning relationship with Joe, Brenda remains emotionally available to Nate. Claire and Russell come into strained, but frequent, contact. Rico and Vanessa mend their relationship just in time for Rico to start to stray. Lisa may be gone, but Nate gravitates toward her more than ever.

If the characters on “Six Feet Under” weren’t so human and disposed of free will, one could be forgiven for thinking a cold, distant satellite was exerting its uncanny force on them. And Nate’s late-night howls alone could plant the idea.

*

‘Six Feet Under’

Where: HBO

When: Sundays, 9 p.m.

Rating: TV-MA (may be unsuitable for children under 14)

Peter Krause...Nate

Lauren Ambrose...Claire

Michael C. Hall...David

Mathew St. Patrick...Keith

Freddy Rodriguez...Rico

Frances Conroy...Ruth

James Cromwell...George

Rachel Griffiths...Brenda

Created by Alan Ball. Executive producers Alan Ball, Alan Poul, Robert Greenblatt and David Janollari.