TONIGHT'S MUST-SEE

"Lost," 9 p.m., ABC. Ever since the survivors ventured into the hatch there's been another mystery: Who is Desmond, the stranger who was living there? And why, lately, has he seemed to know what will happen a little in the future? Tonight, Hurley and Charlie try to get to the truth, and we start to learn about Desmond. Meanwhile, life on "Lost" is almost back to normal. Kate and Sawyer fled the island where they were held captive with the help of the daughter of the Others' leader. Jack remains on the island and remains interested in Juliet — who, it turns out, is a brilliant fertility researcher who was tricked into coming to the island three years ago. All of that happened last week in a fierce episode that also included (via flashback) approximately the world's best hit-by-a-bus scene.

"The Hidden Epidemic: Heart Disease in America," 9 p.m., PBS. From its first minutes, this documentary has our attention. It introduces zestful people who were hit early and hard by heart attacks. Then it looks back: As the 20th century began, it says, heart disease wasn't even in the top 10 among killers of Americans. Now it's the clear No. 1. Indeed women are 10 times more likely to die of heart disease than of breast cancer. The changes during the century were clear. There was more smoking and less exercise; there was more obesity, cholesterol and stress. Undoing that has been difficult. "Hidden Epidemic" traces the efforts in everything from detection to transplants.

OF NOTE

"Go, Diego, Go" (7:30 a.m.), "The Backyardigans" (8:30 a.m.) and "Wow! Wow! Wubbzy" (11:30 a.m.), Nickelodeon. Here are new episodes designed for Valentine's Day. In the first, Sammy the Sloth (who's not terribly quick) must deliver cards by the end of the day. The second has leather-clad bikers racing to return a mailbag. The third has the mailman sick on Valentine's Day; Widget tries to develop a mail-delivering machine.

"Jericho" return, 7 p.m., CBS. Next week, this show starts the second half of its season with a superb episode that tells of the days prior to the nuclear blast. Before that, however, you'll probably need to catch up. Here's a recap episode with highlights from the first 11 weeks.

"Friday Night Lights," 7 p.m., NBC. As the big play-off game nears, Smash is leading a protest: The black players refuse to play unless Mac, the assistant coach, is fired for his racist remark.

"George Lopez," 7 p.m., ABC. Even Benny's lawyer (Adam West) is against her. Now her only hope is the testimony of her disapproving son, George.

"Knights of Prosperity," 7:30 p.m., ABC. Now the team has a real mission, beyond its absurd rob-Mick-Jagger scheme. Esperanza has been kidnapped by her former boyfriend from their old homeland of Colombia.

"In Case of Emergency," 8:30 p.m., ABC. Harry (Jonathan Silverman) has already had bizarre (and funny) misadventures but here's a new extreme. He's supposed to help soothe things between Jason (David Arquette) and his mom. He soon overachieves; now his friend's mom — played by an appealing Jane Seymour — lusts for him. The result is funny and — within the limits of this offbeat comedy —sort of believable.

"Medium," 9 p.m., NBC. Allison wakes up from her dream convinced she's someone else. Her husband tries unsuccessfully to dissuade her.