Searching for Roots, Finding Characters "Family Tree" is a half-hour HBO comedy about a lethargic, unemployed young British man who becomes obsessed with tracking down his ancestors. It's largely improvised and poses as a documentary, though to such a slight degree that it hardly matters. That this does not sound like a terrible idea for a television show is largely because of the past work of Christopher Guest, who, as it happens, created and wrote "Family Tree" with Jim Piddock. Depending on your taste, that's either a tribute or a reproach. Mr. Guest, of course, helped to pioneer what's become known as the mockumentary in films like "This Is Spinal Tap" and "Waiting for Guffman," for which he served variously as writer, director and actor. Having set up the template for TV shows from "The Office" to "Parks and Recreation" to "Modern Family," he comes to television at the risk of looking like a copy of a copy of himself. And there is a familiarity to the series. As in Mr. Guest's films, its story is less important than the presentation of a gallery of eccentrics, ranging from mildly odd to completely loony. Here the narrative thread is even more slender - there's no community play ("Waiting for Guffman") or memorial concert ("A Mighty Wind") - to lend structure. One result is that through the four episodes sent to critics, "Family Tree" can feel a little loose and inconsequential, as Tom Chadwick (Chris O'Dowd of "Bridesmaids") unearths one dotty, inglorious relative after another. But that also means that we get to spend more time with Mr. Guest's crack cast of improvisers - eight episodes in Season 1, with the hope of more - and there are moments in each half-hour that pay off. Some involve Mr. Guest's longtime accomplice Michael McKean, who plays Tom's father, Keith, with poker-faced fatuity. Keith is addicted to terrible 1970s British sitcoms; we get to see snippets of several fake versions of these, including a bawdy cop show and an Anglo-Indian family comedy. The funniest moments almost always involve Tom's highly neurotic sister, Bea, played by the British comedian and ventriloquist Nina Conti, whose stage prop - a monkey hand puppet - has been incorporated into the show. The ever-present Monkey is Bea's therapeutic tool, able to say what she can't: insults, complaints, foul observations. (Bea: "It's inflatable." Monkey: "Like your wife." Bea: "Monkey!") Also notable is Tom Bennett as Tom Chadwick's best friend, Pete, if only because his embodiment of the "Office"-era Ricky Gervais is so uncanny. Carrying the story is the amiable Mr. O'Dowd, whose distinctive accent is explained by Tom's having lived in Ireland as a child. Now 30, with no job or wife, Tom is inspired by the bequest of a trunk of heirlooms to investigate his family as a way of - finding himself? Filling the empty days? It's not exactly clear. Presumably there are epiphanies ahead. If they come, they may take place in America, where the action of "Family Tree" will shift in the second half of the season. The so far quiet and modest vibe of the show may shift as well. In any case, more members of the Guest ensemble, including Ed Begley Jr. and Fred Willard, will appear, which can only be a good thing. Family Tree HBO, Sunday nights at 10:30, Eastern and Pacific times; 9:30, Central time. Produced by Lucky Giant and HBO in association with NBC Universal and the BBC. Created and written by Christopher Guest and Jim Piddock; directed by Mr. Guest; Mr. Guest, Mr. Piddock, Karen Murphy, Deborah Oppenheimer and Mario Stylianides, executive producers. WITH: Chris O'Dowd (Tom Chadwick), Nina Conti (Bea), Tom Bennett (Pete), Jim Piddock (Mr. Pfister), Michael McKean (Keith) and Christopher Fairbank (Neville St. Aubrey).