"B Positive," created by Marco Pennette and airing at 8:30 p.m. Thursdays on CBS under producer Chuck Lorre's banner of mainstream hits, has the distinction (both positive and negative) of being the first new prime-time comedy to premiere in this strange and postponed fall season. With a cheerful dose of dutifulness, the show all but insists that life must go on for all of us. Even the mundane must endure.

Interestingly (sort of), a life-or-death medical matter is at the crux of "B Positive's" premise: Thomas Middleditch ("Silicon Valley") plays Drew, a recently divorced Connecticut therapist who learns that his urinary issues have confirmed the worst: imminent renal failure. Dialysis looms, and Drew's doctor urges him to ask people he knows if they might be willing and able to donate one of their kidneys to him.

At a friend's wedding, Drew meets an old acquaintance from high school, Gina (Annaleigh Ashford from "Masters of Sex" and "Bad Education"), a big-hearted but rambunctious party girl who works as a shuttle driver for an assisted-living facility. Gina offers to be Drew's kidney donor — and a test confirms that she's a good match, on the condition that she can stop drinking and doing drugs for three months.

Gina agrees to try, while Drew heads off to dialysis three times a week, where he meets a very sitcom-like trio of fellow patients — a former pro-football player (Terrence Terrell), a sharp-elbowed corporate striver (Briga Heelan) and an affable dentist (David Anthony Higgins) — who are all tended to by a gay, no-nonsense nurse (Darryl Stephens).

When you throw in Drew's ex-wife (Sara Rue) and 12-year-old daughter, Maddie (Izzy G), as well as Gina's dimwitted best friend (Kether Donohue), "B Positive" suddenly has more characters than the latest season of "Fargo," and barely a chance to focus on its overall story. As Gina moves into Drew's house to work on her sobriety (and elude the loan shark who's after her), it's never clear what sort of outcome we should be hoping for here: true love or surgical success or a combination of both?

Some viewers may ask what they see as the bigger question: How did two actors as talented as Middleditch and Ashford wind up in a mainstream sitcom?

That's actually quite easy to answer: Go ask Melissa McCarthy about "Mike and Molly" syndication royalties and you will wonder no more why it's still a choice gig. And it's not entirely accurate to say that Lorre's shows sand away all traces of edginess; Lorre has perfected a tone that pushes just far enough into raunchy territory that somehow placates network brass while not completely patronizing the viewers.