Channel Surfing: 'Who Do You Think You Are?' "Who Do You Think You Are?," NBC's genealogy-tracing series, doesn't get a lot of attention, but it's one of the most consistently interesting shows on television, and its current season, the third, has been a corker so far. The series, which traces the family trees of celebrities as a way to illuminate history, found a remarkable wrinkle in the ancestry of Martin Sheen in the season opener, and in Episode 2 it helped Marisa Tomei flesh out a family story about a death by gunfire in the distant past. Those installments were pretty absorbing, but Friday's episode, about the actor Blair Underwood, gives them a run for their money in terms of compelling revelations and illuminating anecdotes. Mr. Underwood was on hand for a sneak preview Wednesday night at the Paley Center for Media in Manhattan, along with Lisa Kudrow, an executive producer of the series; Kim Cattrall, who was profiled in Season 2 of the show; and Alex Graham, who created the show in Britain and then helped bring it to the United States. The producers warn participating celebrities that not everything the researchers find in the family tree will be complimentary. "It was very painful," Ms. Cattrall told the crowd Wednesday of her episode's revelations about a grandfather. Mr. Underwood, though, said he was ready and willing. "I said, Give me anything," he told the audience. He got quite a bit. One ancestor is described in a newspaper account as "a Negro religious enthusiast or lunatic." Another branch shows a line of free blacks in Virginia stretching back to well before the Civil War. Mr. Underwood learns about the odd Virginia laws that appear to have led one of his black ancestors to own slaves. He also gets a glimpse of where genealogical research is going as a result of DNA analysis. Such tests have for a while now been able to tell people what part of the world their ancestors came from, but because DNA databases on individuals are now in place, Mr. Underwood gets a much more specific surprise. Josh Hanna of ancestry.com, the genealogical Web site whose research tools are featured in the show, said such connections are likely to become more common. "As the database builds, not just in the United States but in places like Cameroon, more and more of us will be able to make these discoveries," he told the audience.