It can't be easy to learn that one's ancestor is a literal horse's ass. But sad-sack Londoner Tom Chadwick takes such news in stride, again quite literally, as he acquires his great-grandfather's horse costume from a long-ago pantomime show, and after trying the rear end on for size, adds it to his collection of quirky family keepsakes.
HBO's droll-to-the-point-of-precious and occasionally delightful Family Tree (Sunday, 10:30/9:30c) follows Tom on an offbeat personal odyssey into his cloudy lineage. "In our clan, family is what disappears when you're not looking at it," says his retired dad, who keeps busy inventing useless objects like a fan for shoe trees. The dad is played by Michael McKean, who like the rest of the cast often talks directly into the camera, mock-documentary/improvisation style. The casting and the format are two of the more obvious signs that Tree is a Christopher Guest production.