"Family Tree": We never dared hope that Christopher Guest would bring his mockumentary talents to television, but he did and it's wonderful. Sorting through a trunk full of family detritus left him by a distant aunt, Tom Chadwick (Chris O'Dowd in full sad-sack-seductive) begins a quest for his roots. Aided by his father Keith (Michael McKean), his sister Bea (Nina Conti) and her talking monkey puppet and his adorably absurd friend Pete (Tom Bennett), Tom makes a Guestian hero journey throughout England and eventually to Los Angeles, encountering all manner of odd and insightful folk along the way. Sweet, sad, very British (it's co-written by U.K. actor Jim Piddock) and utterly hilarious. HBO, Sunday, 10:30 p.m.