Alyson Hannigan

TONIGHT’S MUST-SEE:

"Veronica Mars," 9 p.m., UPN, Channels 9 and 57.

Alyson Hannigan, the "Buffy the Vampire Slayer" co-star, makes another guest stop as Trina. She wants a favor from her dad (Harry Hamlin), but she has a secret.

"American Idol," 8 p.m., Fox, Channels 5 and 29; "Nashville Star," 10 p.m., USA Network.

Nadia Turner has become this year’s Tamyra Gray or La Toya London. For the third time, a black female singer with immense talent has been ousted before the final two.

Turner is partly to blame for choosing some bland songs. Now the seven survivors are reminded to push hard each time.

Meanwhile, "Nashville Star" is closer to its finale. Last week, it ousted Justin David (a strong instrumentalist with a so-so voice) and had its final four.

Each of those four - Jason Meadows, Erika Jo Heriges, Jayron Weaver and Jody Evans - came up with strong performances. Tonight, we learn which three will be in the finals.

Bret Michaels, one of the judges, also gets another turn on stage. He’ll sing his new country single, "Open Road," plus a song he did with Poison, "Ain’t Nothin’ But a Good Time."

Other choices tonight include:

"NCIS," 8 p.m., CBS, Channels 2 and 3.

Investigators probe a report that a military wife shot a would-be rapist.

"Nova: scienceNow," 8 p.m., PBS, Channels 12 and 13.

Here’s the second of the occasional hours in which "Nova" takes briefer looks at timely topics. One is the discovery of what some scientists consider a separate species - humans, barely 3 feet tall, from 18,000 years ago. Other subjects include stem cells, new dinosaur research and a look at what some frogs - frozen solid each winter - can tell us about cryogenics.

"House," 9 p.m., Fox, Channels 5 and 29.

Last week, this terrific show saw Dr. House almost buckle under pressure from Vogler (Chi McBride), the hospital’s chairman. He didn’t. Tonight, however, they have a showdown.

"Scrubs," 9 p.m., NBC, Channels 4 and 10.

Carla and Turk begin couples’ therapy. Meanwhile, a well-meaning neighbor is being sued by an elderly patient’s brother.

"The Office," 9:30 p.m., NBC, Channels 4 and 10.

A basketball game has a lot at stake: Michael has been trash talking and made a serious bet; Jim is desperate to impress Pam, whose fiance is on the other team.

"Law & Order: Special Victims Unit," 10 p.m., NBC, Channels 4 and 10.

This rerun follows Fin (Ice-T) on an emotional odyssey. That includes a shooting at a convenience store, a reunion with his son and a return to the narcotics squad.