In a TV world where everyone wants to push the envelope, HBO is amping it down.

“Family Tree” (Sun., 10:30 p.m.) is a gentle and effortlessly charming mockumentary from Christopher Guest, the genius behind “Best in Show.”

The show spins around endearing sad sack Tom Chadwick (Chris O’Dowd, above, the lovestruck cop from “Bridesmaids”). Tom has lost his job and his girlfriend; he’s uneager to move forward because dating would “get in the way of my wallowing, which I have really grown attached to.”

In episode one, a great aunt dies, leaving a box of “bits and bobs,” including “an early prototype” Victorian dildo and a photo of a distinguished gent assumed to be Tom’s great-grandfather. Only it turns out to be Prince George; great-granddad was the photographer. In the eight-part series Tom will investigate his forebear and banter wistfully with friends like lifelong pal Pete, who on their first day of school saw that Tom was being teased for wetting himself and told him: “I do it all the time, sometimes on purpose because I enjoy the sensation.”

“With that a bond was formed,” recalls Tom. With that sort of sweet and loopy dialogue, a bond was formed between me and “Family Tree.”