Christopher Guest has spent much of his brilliant career exploring the fine line between stupid and clever, as one his bandmates in “This Is Spinal Tap” put it. The characters in the Guest school of improvised mockumentary comedy are inevitably delusional about something, whether their talent, their level of celebrity, or their romantic life. Yet even though his movies — notably “Spinal Tap” (directed by Rob Reiner but co-written by Reiner, Guest, Michael McKean and Harry Shearer) and his late ‘90s/early ‘00s trio of “Waiting for Guffman,” “Best in Show” and “A Mighty Wind” — feature ridiculous, oblivious people, there’s almost always something real at the heart of their delusions. The band in “Spinal Tap” is a middle-aged joke, but they have enough musical skill that Guest, McKean and Shearer have been able to mount multiple tours as their British alter egos. The community theater musical in “Waiting for Guffman” is terrible, and yet in certain numbers you can see the germ of an idea that could be great if it were given to someone with greater skill than these enthusiastic klutzes.
 
Family Tree,” the new HBO comedy Guest co-created with frequent collaborator Jim Piddock (it debuts Sunday night at 10:30), is something different. It features the usual assortment of Guest-ian foolish dreamers — many of them played by Guest repertory players like McKean, Piddock, Fred Willard and Ed Begley Jr. — and a familiar degree of absurdity. But at its center is a character the likes of which Guest hasn’t had much use for since his very first film, 1989’s “The Big Picture”: a sane, sensible hero.
 
Chris O’Dowd (Kristen Wiig’s cop boyfriend from “Bridesmaids”) plays Tom Chadwick, a young man set adrift after he loses his job and long-term girlfriend at the same time. When an elderly aunt dies and bequeaths him a trunk full of family mementos, Tom tries to distract himself from his recent misfortune by tracing his ancestry. He discovers a great-grandfather with an embarrassing niche in local theater, a grandfather who boxed in the 1948 Olympics, and an entire branch of the family tree he never knew existed, in part because his father Keith (McKean) is a man of few words on any subject but his favorite television show: a corny,‘70s police sitcom called “Move Along, Please.”(*)
 
(*) Because this is a somewhat more earnest story than the kinds he usually tells, Guest gets to exercise some of his satiric muscles with the various terrible fictional TV shows Keith adores, most of them seeming like entirely plausible entries of their respective genres. (I actually had to Google “Move Along, Please” and its slow-burning star Richard Breen to confirm that they weren’t real.)  
 
Along this journey, Tom encounters lots of ridiculous situations and people — and frequently brings along one of the latter in his best friend Pete (Tom Bennett), a cheerful sort with an over-inflated sense of his abilities at pretty much everything — but he remains a very self-aware, human character. O’Dowd is the best sort of straight man, in that he’s able to be funny reacting to the madness around him, but it’s a different sort of character, and storytelling, than Guest fans may be used to.
 
“Family Tree” is gentler and more empathetic than Guest’s usual stock in trade. Even a completely outlandish character like Tom’s sister Bea (actress/ventriloquist Nina Conti), who began using a monkey hand puppet to deal with a childhood trauma (albeit an absurd one) and has never been able to let go of it, is presented with some level of sadness. The show is always aware that Monkey’s blunt, foul-mouthed presence has ruined Bea’s adult life, even as it wrings enormous laughter out of the furry guy’s antics.
 
And that’s perhaps the most impressive thing about “Family Tree”: it invites you to laugh at all these kooks in a way that doesn’t feel mean-spirited, and it takes parts of its hero’s journey quite seriously. There are sequences where Tom is learning something about a long-dead relative that are completely sincere in their interest in old-time theater or the ’48 “Austerity Games.” And then there are sequences where one of Tom’s long-lost relatives invites him to castrate a lamb or Monkey is behaving very inappropriately at a Greek wedding.(**)
 
(**) The latter scene is among the most uncomfortable of the entire Guest oeuvre, and this is a guy who was trafficking in the comedy of discomfort long before Larry David and Ricky Gervais got rich doing it.
 
This balance of heartfelt emotion and absolute lunacy is probably the only way a Guest project could work as an ongoing series. As much as I love “Guffman,” a show built around Corky St. Clair would get old very quickly. Tom Chadwick is a man with a fixation that’s both reasonable and relatable, and he becomes our tour guide to the familiar, funny Christopher Guest worldview.
 
Alan Sepinwall may be reached at sepinwall@hitfix.com