Li'l sis tags along on a slaying mission on Buffy the Vampire Slayer (8 p.m., WB). That '70s Show (8 p.m., Fox) opens a new season with a parody of "Reefer Madness." Jerry Orbach, star of stage, screen and "Law & Order," is the subject of a new Biography (8 p.m., A&E). JAG (8 p.m., CBS) begins its new season with a two-parter steeped in Russian intrigue and military corruption. James "King of the World" Cameron is the executive producer and co-writer of Dark Angel (9 p.m., Fox), a new futuristic series about a babelicious young woman who happens to be an escapee from a secret government lab. Jessica Alba and John Savage star in the two-hour pilot that Fox is airing instead of the presidential debate. On Angel (9 p.m., WB) (he's not dark, just depressed), our hero has a sense of dij` vu when he investigates a strange presence in an abandoned hotel.
Democrat Al Gore and Republican George W. Bush face off in their long-awaited first presidential debate (9 p.m. EDT/6 p.m. PDT, ABC, CBS, PBS, CNN, C-Span, MSNBC, Fox News). Note: NBC is giving local affiliates a choice of airing either the debate or the American League Division playoff game live; check local listings. In the new five-part series Building Big (8 p.m., PBS, check local times), author David Macauley shows us how some of the world's greatest feats of engineering -- bridges, skyscrapers, tunnels, etc. -- came to be.
Braves at Cardinals, Game 1 (1 p.m., ESPN)
Mariners at White Sox, Game 1 (4 p.m., ESPN)
Yankees at A's, Game 1 (8 p.m., NBC. NBC is giving local affiliates a choice of airing either the game or the presidential debate live; check local listings)
Rosie O'Donnell (syndicated) Regis Philbin, Joshua Jackson
David Letterman (CBS) Christine Baranski, Green Day
Jay Leno (NBC) Yasmine Bleeth, Jackie Chan, Phish
Politically Incorrect (ABC) Gore Vidal, Michael Moore
Conan O'Brien (NBC) Magic Johnson, Michael Rapaport
Craig Kilborn (CBS) Sylvester Stallone
All times Eastern unless noted.