As “The Shield” (9 p.m., FX) enters its sixth season, crooked cop Vic Mackey (Michael Chiklis) is still searching for the murderer of his friend and confederate Lem. But fans of the violent and gripping series know that he’s looking in all the wrong places.

And while Vic seeks vengeance, his nemesis from Internal Affairs, Jon Kavanaugh (Forest Whitaker), is out to take him down, even if it takes breaking a few rules.

The bitter rivalry between Mackey and Kavanaugh is the central drama in this consistently surprising series, and the chance to watch Emmy-winner Chiklis and Emmy- and Oscar-winner Whitaker tangle should be more than enough to keep “The Shield” on your must-see list.

¢ Legendary singer Tony Bennett arrives to mentor the remaining nine contestants on “American Idol” (7 p.m., Fox). While Bennett is considered a classic interpreter of standards from the American songbook, he was the quintessential pop artist when he was signed to Columbia Records in 1950. Sure, he could sing jazz-inflected tunes like his fellow Italian American crooners, but one of Bennett’s early big hits was his interpretation of “Cold, Cold Heart” by Hank Williams. The would-be “Idol” winners will have to show similar genre-bending versatility if they are to survive.

¢ “Deadliest Catch” (8 p.m., Discovery) returns for a third season. This documentary series about the lucrative and dangerous life of crab fishermen working the icy waters near the Bering Strait has won one of the largest and most faithful audiences on cable television.

¢ “Nova” (7 p.m., PBS, check local listings) looks at cuttlefish, masters of disguise that leave chameleons in the dust. Known as “kings of camouflage,” cuttlefish have been known to masquerade as the opposite sex and even hypnotize their prey. How do they do it? Their brain-to-body ratio is off the charts.

Tonight’s other highlights

¢ Scheduled on “Dateline” (7 p.m., NBC): the search for an identity theft.

¢ Richard goes to the hospital on “Gilmore Girls” (7 p.m., CW).

¢ The women’s NCAA basketball tournament (7:30 p.m., ESPN) concludes.

¢ The rescue of a downed plane takes the crew to New Guinea on “The Unit” (8 p.m., CBS).

¢ A murdered doctor may have snitched on a colleague on “Law & Order: Criminal Intent” (8 p.m., NBC).

¢ A pregnant paparazzi faces a painful choice on “House” (8 p.m., Fox).

¢ The “Frontline” (8 p.m., PBS, check local listings) film “So Much, So Fast” looks at ALS (“Lou Gehrig’s Disease”) through the prism of a 29-year-old Boston man and his supportive family.

¢ An A-student fails to survive a college party on “Law & Order: Special Victims Unit” (9 p.m., NBC).

¢ An angry plaintiff takes the firm hostage on “Boston Legal” (9 p.m., ABC).

¢ “China Blue” on “Independent Lens” (9 p.m., PBS, check local listings) follows a single pair of denim jeans from a manufacturing plant in China to a big box store in the United States to show how global trade affects workers and consumers all over the world.