TONIGHT'S MUST-SEE

"American Masters: Walter Cronkite: Witness to History," 9 p.m., PBS. In an era when three-fourths of all families watched one of the three network newscasts, Walter Cronkite shaped history. It's a fascinating story, richly told by Cronkite (who will turn 90 on Nov. 4) and people who seem to adore him. It's narrated by Katie Couric, who in six weeks takes the anchor seat that Walter Cronkite and Dan Rather held for more than four decades.

"30 Days" season-opener, 7 p.m., FX. Frank George was 7 when he and his family fled the post-revolution Cuba, leaving everything behind. Today, at 56, he's a U.S. citizen, an electronics technician and a staunch opponent of illegal immigration; as a member of the Minutemen, he sometimes packs a pistol and patrols the border. Now he spends 30 days sharing the life of an illegal-immigrant family. The seven people live on $15,000 a year in a one-bedroom, 500-square-foot apartment while sharing American dreams. The eldest daughter competes in high school golf tournaments and nervously awaits news of her application to Princeton. The month becomes an emotional experience for George and for viewers.

OF NOTE

"Homeless to Harvard: The Liz Murray Story" (2003), 4 p.m., Lifetime. In a way, this stirring movie neatly matches the dreams of the "30 Days" family. In real life, Liz Murray was often on her own, sleeping and studying in stairwells. She became a top student and went on to Harvard. Thora Birch and Kelly Lynch are superb as Birch and her mother.

"Nightmares and Dreamscapes: From the Stories of Stephen King," 6 and 7 p.m., TNT. The first hour has Tom Berenger as a novelist who buys a creepy painting, then is terrorized by it. The second hour avoids any supernatural elements. Jeremy Sisto is terrific as an ex-convict, pulled into a new scheme during his first day of freedom.

"America's Got Talent," 7 p.m., NBC. The semi-finals continue. Judges will choose one finalist and viewers choosing another.

"So You Think You Can Dance," 7 and 8 p.m., Fox. First, we get a rerun of last week's episode; then the final eight dancers perform.

"Project Runway," 7 p.m., Bravo. Here's one more chance to catch up. The season's two episodes rerun at 5 and 6 p.m., before this third one arrives at 7.

"Secrets of the Dead: The Sinking of the Andrea Doria," 8 p.m., PBS. Tuesday (July 25) marked the 50th anniversary of the day the Italian luxury liner Andrea Doria collided with the Swedish ship Stockholm. Quick rescue actions saved more than 1,600 people with only 51 killed.

"The Aristocrats," 8 p.m., HBO. This strange film has 100 people — top comedians, mostly — talking about and even re-telling the same nasty joke.

"Primetime Medical Mysteries" debut, 9 p.m., ABC. This is a documentary that might also appeal to "House" fans with doctors facing perplexing situations. The opener involves sleep disorders.