Given his history, it’s a wonder Christopher Guest and HBO haven’t hooked up sooner, what with the cultish devotion to his understated satires "Best in Show" and "A Mighty Wind." Yet the actual wedding, "Family Tree," bears rather disappointing fruit, so droll and dry as to be nearly taste-free, with only the occasional hint of bite. Hamstrung in part by a slim premise, the show possesses star power with Chris O’Dowd as its searching protagonist, but with apologies to Guest’s beloved "This Is Spinal Tap," on a scale of 1 to 10, this British-U.S. collaboration is about a 5.

O’Dowd plays Tom Chadwick, a Londoner recently dumped by both his girlfriend and employer, whose father (longtime Guest collaborator Michael McKean) presents him with a box from Tom’s deceased great aunt. In it, he finds a picture of his great-grandfather Harry, which sets him upon a journey to learn all he can about this mysterious ancestor. That includes meeting long-lost relatives (the eight-episode run eventually shifts to the U.S.), augmenting Tom’s quirky stable of existing kin.

If only there were any consistency in the writing (by Guest and Jim Piddock, with the former directing), which includes mockumentary-style direct-to-camera interviews, a device that in this context doesn’t make much sense.

Guest’s approach is as much about creating atmosphere and fostering discomfort as it is about belly laughs, but there’s a difference between being droll and positively arid.

Granted, just adding Guest to HBO’s arsenal of auteurs is something of a coup, given his audience, and this latest BBC partnership doubtless comes with certain financial benefits. But while this foray into series territory possesses the same look and feel as Guest’s proven mocu film formula, the result merely demonstrates that it’s possible for the "Tree" to fall pretty far from the apple.

"Family Tree"

Series; HBO, Sun. May 12, 10:30 p.m.

Credits: Filmed in London and L.A. by Lucky Giant and HBO in association with NBCUniversal and the BBC. Executive producers, Christopher Guest, Jim Piddock, Karen Murphy, Deborah Oppenheimer, Mario Stylianides. running time: 30 MIN.

Cast: Chris O’Dowd, Nina Conti, Tom Bennett, Michael McKean, Lisa Palfrey, Jim Piddock.