TONIGHT'S MUST-SEES

"Back to You," 7:30 p.m., Fox. At the end of a sometime-hilarious episode last week, Kelly (Patricia Heaton) relented. She promised to tell her daughter that Chuck (Kelsy Grammer) is the girl's father. Or will she? Tonight, Kelly backs down — especially after a therapist raises new issues about the relationship between these two. It's a promising round for what has been one of the year's best new comedies.

"Secrets of the Dead: Aztec Massacre," 8 p.m., PBS. Historians often describe the Spanish conquistadors as brutalizing native tribes in the Americas. Now comes a much richer, more-varied portrait. A mass grave in Mexico reveals that European men and women were killed and mutilated. That leads to this portrait of evils on both sides — from the Spaniards' deception to the Aztec practice of ripping a still-beating heart out of an enemy's body. It's a fascinating (but gory and grotesque) hour.

OF NOTE

" 'Til Death," 7 p.m., Fox. After Joy has a pregnancy scare, Eddie agrees to have a vasectomy. Then he confers with his new pal Kenny (J.B. Smoove).

"American Idol," 8 p.m., Fox. Two weeks ago, viewers were shocked to see Michael Johns, a front-runner, voted out. Last week, they weren't shocked to see Kristy Lee Cook ousted, after spending most weeks in the bottom three. Now we'll see who stumbled during Andrew Lloyd Webber week.

"Strange Days on Planet Earth," 9 p.m., PBS. This intermittent National Geographic series, hosted by Edward Norton, looks at crises in the water. In the first, overfishing endangers people in Ghana and Namibia. In the second, water brings health crises to humans and animals.

"Law & Order," 9 p.m., NBC. It's a pivotal episode for this solid cop show. Jesse L. Martin is leaving after nine strong years as Detective Green. Anthony Anderson — who has been superb in everything from comedies to "The Shield" — will take over. We meet him tonight as he investigates Green.