* The Witness: From the Balcony of Room 306 (8 pm/ET HBO2)
For years, Rev. Samuel "Billy" Kyles wondered why he alone was with Dr. Martin Luther King Jr. on the balcony of the Lorraine Motel in Memphis when he was assassinated on April 4, 1968, at the age of 39. By the end of this stirring, Oscar-nominated 2008 documentary, you'll hear Kyles' conclusion. Clocking in at just over 30 minutes, the film tells the story of King's support for the strike by the city's sanitation workers, and shows footage of what turned out to be the last of his many memorable speeches.
* Lost (9 pm/ET ABC)
We don't know yet how many of the Oceanic 6 will decide to return to the island. (Anyone care to bet that it won't be all of them? Didn't think so.) But at least we'll find out tonight how they'll get there, courtesy of Daniel Faraday's elegantly creepy church-lady mother, Eloise Hawking (Fionnula Flanagan). And back on the island, will we see if Locke's wheel-realignment job last week stopped those time flashes? Another question: How will Locke's "sacrifice" play into all of this?
* American Idol (8 pm/ET Fox)
Life goes on for our 36 sultans of sing. It's been sort of like a showbiz passage of natural selection: These folks were the fittest to survive, outlasting the preliminary rounds, then the tense group auditions, then the solo showcases, then the grueling cuts at the judges' mansion. Last night, the first 12 of the Top 36 semifinalists performed, and viewers voted for the first time this season. Tonight this Darwinian evolution continues when the results of those votes are revealed. Three finalists will be chosen - while nine hopefuls are eliminated.
* Spectacle (9 pm/ET Sundance)
Host Elvis Costello takes a back seat on an episode that features his wife, Canadian jazz pianist and singer Diana Krall. Elton John conducts the playful interview, which includes a discussion of Krall's favorite singers and her need to explore the work of other artists. The two also perform John's "Sorry Seems to Be the Hardest Word." Later, Elvis shows up and joins in for a memorable closing number.
* Top Chef: New York (10 pm/ET Bravo)
It's time for the main course. The final four chefs - Fabio, Hosea, Carla and Stefan - battle it out in Part 1 of the two-part Season 5 finale, a supersize food fight that features a scene switch from New York City to the ragin' Cajun capital, New Orleans. It won't be easy in the Big Easy: There will be a sizzling reduction from four chefs to three, and they'll need to kick it up a notch to impress the precise palate of celeb chef and guest judge Emeril Lagasse.