It was one of those moments that can only be conveyed on television, as David Letterman revealed on his Thursday night show that he had been the target of a blackmail plot over his sexual relationships with female members of his staff.

Reading Letterman's words offer the information about it, but not the emotional impact of listening to him jokingly describe both the plot and his own behavior. It came after a monologue when Letterman again tweaked South Carolina's adulterous governor, Mark Sanford, for "hiking the Appalachian trail."

Letterman said that the alleged blackmailer -- identified by the New York Daily News as Robert Halderman, an Emmy-winning producer at CBS' "48 Hours" -- said he was planning to write a screenplay revealing the "creepy things" done by the late-night talker.

"The creepy stuff was that I have had sex with women who work for me on this show," Letterman said.

There was the usual Letterman smirking, and the laughter from an audience that seemed to move slowly from thinking it was all a comedy bit to understanding that this was a serious matter. The studio audience did gasp when he said he was asked by the extortionist to pay $2 million.

Expect plenty of talk about what this will do to Letterman's ratings.

This mess exploded just as Nielsen Media Research numbers came out this week showing Letterman's "Late Show" scored well over NBC's "Tonight Show" last week, technically the first week of the 2009-'10 season, averaging more than 5 million viewers. Most importantly, Letterman beat Conan O'Brien among viewers 18-49 and 25-54, and the two were tied among viewers 18-34.

NBC has been expecting O'Brien's "Tonight Show" to win over the 62-year-old Letterman among younger viewers.

Unless one or more of Letterman's female staff members (and we don't know how many of them are involved) decide to sue him over the affairs, don't expect much impact on the show.

Remember the last time Letterman made headlines?

It was a joke he told in June about Sarah Palin and her 18-year-old daughter, Bristol. It led to misunderstanding and an eventual on-air apology. It also led to talk that it would hurt his ratings.

But that kind of talk generally comes from folks who wouldn't watch Letterman in the first place.

ON TV: TLC has suspended production on "Kate Plus Eight," as the show is now known, while it deals with Jon Gosselin's claim that the cable channel could no longer film the children at the family home he no longer lives in.

• Ryan Reynolds and Lady Gaga headline this weekend's "Saturday Night Live" at 10:30 p.m. Saturday on Channel 4. But all eyes will be on Jenny Slate, the new cast member who dropped an f-bomb on the season opener.
• NBC was planning for a 13-episode sci-fi thriller called "Day One" to start airing after next year's Winter Olympics. But the network has cut it back to a four-hour miniseries.
• Who says soaps are dead? James Franco has signed on for a couple months on ABC's "General Hospital."
• The fall network TV lineup continues to roll out, with CBS' medical drama "Three Rivers" launching at 8 p.m. Sunday on Channel 58.

SOMETHING BIG ABOUT NOTHING: The big TV event of this weekend is the return of the old "Seinfeld" crew to prime time along with Larry David on HBO's "Curb Your Enthusiasm" at 8 p.m. Sunday.

The five-episode "Seinfeld" plot features the hapless Larry trying to reassemble the old cast in an attempt to win back his ex-wife.

Most interesting to see will be the performance from Michael Richards, who hasn't been very public since he was captured on cellphone video in a 2006 tirade using racial epithets.