Secrets arise when Gibbs' former mother-in-law surfaces as a witness in a murder investigation.

The Top 10 female semifinalists perform live for America's vote. Tune in to see who sizzles and who fizzles. Performing Tonight Are: Crystal Bowersox Didi Benami Haeley Vaughn Katelyn Epperly Katie Stevens Lacey Brown Lilly Scott Michelle Delamor Paige Miles Siobhan Magnus

This week's first challenge is a temptation with, literally, hundreds of cookies. Ali tells the teams that whoever wins this temptation challenge will get a huge reward: they'll be able to divide everyone into the two traditional teams, Blue and Black. Also, they could give one contestant immunity from this week's elimination. Let the games begin!

When the American Museum of Natural History's Hayden Planetarium stopped calling Pluto a planet, director Neil deGrasse Tyson found himself at the center of a firestorm led by angry Pluto-loving elementary school students. What is it about Pluto-a cold, distant, icy rock-that captures so many hearts? Four years after the IAU (International Astronomical Union) officially reclassified the ninth planet as a plutoid, NOVA travels cross-country with Tyson to find out.

Dogen asks Sayid to make a decision, while Locke sends Claire to give the temple inhabitants a warning. Meanwhile, in another life, Sayid must come to the aid of his family and meets a familiar face.

The death of a lieutenant commander appears to be suicide but the NCIS team uncovers evidence corresponding to murder.

Also, Operational Psychologist Nate Getz joins Special Agent Kensi Blye in the field.

In the Masonic Lodge a man is murdered. When Murdoch discovers the man is leading a double life, a member of Murdoch's team goes undercover to discover the murderer.

The possible abduction of an elderly man by a known drug addict; a freeway shooting; Chickie's new partner shoots a 13-year-old boy.

While Peter gets acclimated to life at home and works on a comeback strategy with his political team, including Eli Gold, Alicia and Diane wrestle with spousal privilege laws in a murder case that hinges on their ability to get a wife to testify against her husband.

Single mom Sarah Braverman moves back home, stirring up the chemistry of her colorful extended family.

When the daughter of a man from his past goes missing, Neal volunteers to help find her. Little does anyone realize the kidnapper is an old enemy of Neal's, and is leading him right into a trap.

The academic performance of Alex and some of his teammates comes under scrutiny as result of dismal grades and GPA's. They eventually resort to providing sexual favors for the professor's 80-year old mother to avoid failing off the team.

When her hard-to-please mom comes for a visit, Krista lies about being the manager of Players to impress her.

Guest: Actor Robert Pattinson, star of "Twilight".

David Brooks

Author Mitt Romney; actress Mia Wasikowska.

Sarah Palin; Olympic Gold Medal Snowboarder Shaun White; musical performance by Adam Lambert.