TONIGHT'S MUST-SEE

"Las Vegas" season-opener, 8 p.m., NBC. When we last saw the show, Delinda (Molly Sims) was supposed to be marrying Derek (Shawn Christian), the handsome doctor. Then she hesitated. Tonight, she admits to Derek that she also has feelings for Danny (Josh Duhamel). Meanwhile, her dad Ed (James Caan) confronts an armed robber. The story will have people wondering if every show must now have a Dr. Derek. That's worked well on "Grey's Anatomy." There's also something odd about a Delinda-Derek-Danny triangle. Should we assume the show will be adding a Debbie, Doug and Donny?

"Return to Halloweentown," 8 p.m., Disney Channel. Here's the fourth movie in this thoroughly successful series. Marnie Piper is now played by Sara Paxton, 17. She's at Witch University, with her friend Aneesa (Summer Bishil). Debbie Reynolds continues to play Aggie Cromwell.

OF NOTE

"Little Einsteins: The Birthday Machine," 8 a.m., Disney Channel. An OK story has the kids flying through old Italy. At least the sights and sounds are kind of nice.

"Battlestar Galactica," 6 p.m., Sci Fi Channel; reruns at 8. Here's a pivotal episode, the fourth in a superb five-parter to launch the season. With only limited support from space hardware, the humans make their daring escape attempt.

"1 vs. 100," 7 p.m., NBC. After debuting last week at 9 p.m., this light and entertaining game show slides into its regular slot. It wrapped up last week with Christine Tarbet of Youngstown, Ohio, in a strong position, still holding her two options for getting help. As tonight's show proceeds, members of "the mob" will range from brain surgeons and geniuses to cheerleaders and members of the Los Angeles Clippers basketball team.

"Trading Spouses: Meet Your New Mommy" season-opener, 7 p.m., Fox. Here's a one-night break from baseball (barring weather postponements), before the World Series begins Saturday. Fox uses the interlude to return this reality show. A Kentucky woman adjusts to life in an Orthodox Jewish family. And a Boston woman adjusts to hunting raccoons in Kentucky.

"Men in Trees," 8 p.m., ABC. Ever Carradine, who was the presidential aide in "Commander in Chief," arrives as Marin's sister. As soon as she reaches this little Alaskan town, there's trouble — including a fire and her interest in Jack.

"Close to Home," 8 p.m., CBS. When a strong businesswoman is killed, her husband becomes a prime suspect.

"Numb3rs," 9 p.m., CBS. Seven people have been attacked on the freeway, with four of them killed. Still, the method seems to keep changing; Charlie tries to find a pattern.

"Law & Order," 9 p.m., NBC. A TV show works to reveal suspected pedophiles. The crisis comes when one of the suspects is killed.