“Will & Grace” (7:30 p.m., NBC) celebrates its 100th show with an hourlong episode. This being sweeps, it’s a Very Special hour, revolving around Grace and Leo’s impulsive decision to get married. Last week Leo (Harry Connick Jr.) used the “L” word, so why wait?

It’s always a tad disappointing when smart shows employ hackneyed sweeps stunts like quickie weddings. After all, do we really expect them to go through with it? Or for Leo to stick around? Give the writers of “Will & Grace” credit for winking at the very notion of sweeps stunts. Grace and Leo tie the knot in a Central Park ceremony that is part of a ratings gimmick for “The Today Show.” Host Katie Couric guest stars, as herself. This marks the second example of symbiosis between “Will & Grace” and the morning news/talk program. Fans may recall an episode some years back when Will and Jack exchanged an impromptu smooch in front of a startled Al Roker.

After hitting its century mark, “Will & Grace” glances back with a clip show retrospective (8:30 p.m., NBC), complete with outtakes, bloopers and never-before-seen moments.

• Leave it to “Frontline” (9 p.m., PBS) to get to the bottom of the most pressing story of the moment: What the heck ever happened to al-Qaida? While network news offers soundbites and cable news channels spew hot air, this smart news magazine consistently delivers the goods. If you want to be informed about the new World War we have embarked upon, watch this show.

Correspondent Martin Smith travels to Pakistan, Yemen, the Gulf of Oman and Saudi Arabia to find out what happened to the al-Qaida terrorist organization in the months after the Taliban were toppled in Afghanistan.

To many, al-Qaida is more than a terror organization or military enemy. “It’s an idea about how to hurt the West,” says Smith. “And it’s an idea that is taking hold with more and more people in countries as disparate as the United Kingdom and Yemen.”

Smith put himself at considerable risk to compile this report. He travels to an area on the Pakistan-Afghan border that many Pakistani journalists find extremely dangerous. In an interview with Smith, Pakistan’s president, Pervez Musharraf, admits that the border with Afghanistan has been all but ungovernable for 200 years.

After Pakistani soldiers forbid Smith to travel any further, he hired a reporter from the local Pashtun tribe and supplied him with a camcorder so he could capture footage of battles between Pakistani special forces and al-Qaida holdouts. “In Search” also covers a four-hour gun battle in the Pakistan city of Karachi that resulted in the capture of Ramzi bin al-Shibh, one of the plotters of the Sept. 11 attacks.

Smith also interviews two rather reticent Saudi princes, and several figures who explain Yemen’s special place in the world of Osama Bin Laden and other Islamic radicals. Many believe that Bin Laden is hiding there. In fact, Bin Laden once told a reporter, “the choice is between Afghanistan and Yemen. The geography of Yemen is mountainous and its tribespeople are armed. It allows one to breathe clean air without humiliation.”

Tonight’s other highlights

• The Peanuts gang puts on a holiday pageant in the 1973 animated special “A Charlie Brown Thanksgiving” (7 p.m., ABC).
• Joshua Jackson and Kate Hudson star in the 2000 thriller “Gossip” (7 p.m., WB)
• The honey-obsessed bear and his pals celebrate the holidays on “A Winnie the Pooh Thanksgiving” (7:30 p.m., ABC). Not to be a nitpicker, but doesn’t Winnie the Pooh live in England, where they don’t celebrate Thanksgiving?
• A performer is murdered during the making of a porn film on “CSI” (8 p.m., CBS).
• Scheduled on “20/20” (8 p.m., ABC): Barbara Walters interviews the Osbournes.
• Sally Field reprises her roles as Abby’s bipolar mother on “ER” (9 p.m., NBC).
• Scheduled on “Primetime” (9 p.m., ABC): video diaries of overweight children; a Texas jailer abuses his female inmates; a profile of Eminem.

Series notes

A 10th contender is ostracized on “Survivor: Thailand” (7 p.m., CBS) : Rachel’s selfish sister (Christina Applegate) ruins the holidays on “Friends” (7 p.m., NBC) : An hour of amateur acts on “30 Seconds to Fame” (7 p.m., Fox) : Wrestling on “WWE SmackDown!” (7 p.m., UPN) : People behave foolishly on “Stupid Behavior Caught on Tape” (8 p.m., Fox) : An Arab medical intern disappears after two personal setbacks on “Without a Trace” (9 p.m., CBS).

Late night

Ice Cube and the cast of the Broadway musical “Hairspray” appear on “Late Show with David Letterman” (10:35 p.m., CBS) : Jay Leno hosts Sean Hayes, Tom Ridge and Norah Jones on “The Tonight Show” (10:35 p.m., NBC).