Tom Bennett and Chris O'Dowd in HBO’s “Family Tree.” (Ray Burmiston /HBO)
TV critic

Sooner or later, often in response to a personal crisis or a birthday ending in a zero, a relative gets the genealogy itch and off he goes, vanishing down the bottomless hole of Google and searches, returning occasionally to breathlessly report census mentions and ocean crossings. Before you know it, there’s a road trip to visit some cemeteries. Great fun, except when it isn’t.

“Family Tree,” HBO’s wobbly yet tender new comedy co-created by Christopher Guest, both honors and satirizes the impulse to know more about the relatives who came before us — those strangers who stare out at us from faded photographs and stained tintypes.

Employing some of the same comic tone of his work on memorable mockumentaries (“This Is Spinal Tap,” “Waiting for Guffman,” “Best in Show,” “A Mighty Wind,” “For Your Consideration”), Guest has assembled a worthy and adept ensemble of oddballs. But it remains to be seen if the story itself will catch on.

Like much of what HBO launches these days, “Family Tree” (premiering Sunday night) takes more than a few episodes to bear fruit. Chris O’Dowd (“Bridesmaids”; “Girls”) stars as Tom Chadwick, a Londoner who is compelled to start looking into his family history when a great aunt he hardly knew, Victoria, leaves him a box of random keepsakes — mostly rubbish — after her death.

Tom, who’s been recently dumped by his girlfriend and downsized out of a job, finds hope in the idea that these objects mean something. This is always the soothing discovery of the genealogy nut, who realizes the weight of history. Generations come and go, and our biggest problems and griefs are just a mere stitch in the tapestry.

Chris O’Dowd stars in “Family Tree,” the new series co-created by Christopher Guest, who is known for mockumentaries such as “Best in Show” and “This Is Spinal Tap.” (Mark Griffiths /HBO)

Tom’s father, Keith (Michael McKean), lives in the suburbs and obsessively watches old sitcoms — one of the best things about “Family Tree,” at this stage, is its deft lampooning of British TV — and he exhibits very little interest in old family history, remembering little beyond his own parents and grandparents.

Undeterred, Tom starts from scratch. Aided by his hyperactive friend Pete (Tom Bennett, playing a character straight from the book of Ricky Gervais), Tom learns about his great-grandfather’s acting career as the back half of a vaudeville-era horse costume. The first four episodes take place in England, until Tom journeys to the United States to pursue information about an expatriate branch; here, one presumes, we’ll encounter characters played by actors who’ve worked with Guest before — Ed Begley Jr., Bob Balaban, Fred Willard, etc.

The mockumentary format feels unfortunately shopworn this time around, used mainly as an expository crutch. This is partly because viewers have grown accustomed to the genre, which has been executed more richly on TV in projects such as “The Office” and “Modern Family.” Guest and company, including co-creator Jim Piddock, adhere to the boundaries of what would be a faux-documentary style only when it suits their narrative purpose; in most cases, scenes are shot from multiple angles simultaneously, forgetting the limitations that a true documentary crew would encounter. This matters only if it matters to you; if it does, it will go on mattering to a distracting degree.

O’Dowd is an engaging lead, even if he’s playing the same sort of guy in almost everything we’ve seen him in so far. Overall, there’s not a lot here that’s gut-bustingly funny, certainly not in the way that Guest’s fans might expect, or with much memorable dialogue or laugh lines I can reproduce for you here.

Instead, “Family Tree” is at its most wry when Tom must process the absurd period details he learns about his forebears. These oddities manifest themselves in bizarre photographs, awkward letters or even as objects of unknown purpose — as when an antique shop dealer cautions Tom, who thinks he’s blowing into a musical instrument, not to put his mouth on a Victorian-era sex toy. Sometimes the discoveries are more embarrassing, as when visiting long-lost relatives only to realize that redheads at the dinner table are descended from an illicit love affair and, therefore, not as related as one hoped.

Identity is the overall theme; if “Family Tree” means to feel as adrift and disoriented as Tom does, then I suppose it’s succeeding in that regard. I’m reminded of David O. Russell’s 1996 film “Flirting With Disaster,” in which Ben Stiller played an adopted man on a madcap search for his birth parents — but only a little bit, because that movie was far more hilarious. “Family Tree” plays off the same general notion: You can hunt all you want, but you might cringe at what you find.

Guest is still an expert at populating the margins of his mockumentaries with unforgettable characters. The best discovery in “Family Tree” is Tom’s neurotic sister Bea, played by comedian/ventriloquist Nina Conti. During intensive therapy as a child, Bea learned to express herself through a monkey puppet named Monk, who has remained on her right hand ever since. Monk acts as her droll, unedited subconscious and is treated like a normal member of the family even though most of what he says is either filthy or shockingly inappropriate. (Conti and Monk appeared briefly in “For Your Consideration” as a news channel’s local weather team.) When Monk speaks, people listen — as well they should. He’s easily the best in show.

Family Tree

(30 minutes) premieres Sunday

at 10:30 p.m. on HBO.