Before competing for viewers with their new fall seasons, the networks join forces to air the “Stand Up to Cancer” (7 p.m., ABC, CBS, NBC, Fox, VH1, E!, HBO, Discovery Health, MLB Network and Style) fundraising event. In the spirit of cooperation, all three network news anchors, Katie Couric, Diane Sawyer and Brian Williams, share hosting duties for this event, airing commercial-free.

Musical guests scheduled to perform include Natasha Bedingfield, Neil Diamond, The Edge, Martina McBride, Aaron Neville, Dave Stewart and Stevie Wonder.

Half the fun of these events is watching the bold-faced names working the phone bank. Look for volunteers, including George Clooney, recently feted for his humanitarian work on the Emmy Awards. Others include Kathy Bates, Dorothy Hamill, Anne Heche, Randy Jackson, Rob Lowe, Marlee Matlin, Gwyneth Paltrow, Will Smith and Denzel Washington.

The first “Stand Up to Cancer” aired in September 2008 and raised more than $100 million. It took place just before the Wall Street maelstrom and meltdown that sparked our enduring economic downturn. So it will be interesting to see how much money gets raised tonight and how that compares to the pre-crash total.

• Barbara Walters returns to “20/20” (9 p.m., ABC) to report on a strange and rare disease.

There are only an estimated 68 cases of Progeria in the entire world, but few victims of the rapid-aging syndrome live past 13.

Progeria causes children to age at 10 times the normal rate, making them look like 50 by their 5th birthday. Walters interviews one of the oldest living children with the affliction and discusses what her body might tell us about both progeria and the aging process.

Tonight’s other highlights

• A juvenile victim fuels Mac’s sense of outrage on “CSI:NY” (8 p.m., CBS).

• Gut instincts prevail on “The Good Guys” (8 p.m., Fox).

• Hidden cameras capture ethical dilemmas on “Primetime: What Would You Do?” (8 p.m., ABC).

• Carter and Grant fight against million-to-one odds on the mid-season finale of “Eureka” (8 p.m., Syfy).

• Robin Williams stars in the disturbing and provocative 2009 dark comedy “World’s Greatest Dad” (8 p.m., Showtime), written and directed by Bobcat Goldthwait.

• After a cry for help, a girl is kidnapped and her mother slain on “Flashpoint” (9 p.m., CBS).

• Shape-shifters crash Audrey’s birthday party on “Haven” (9 p.m., Syfy).

• The spoof miniseries “The Kids in the Hall: Death Comes to Town” (9 p.m., IFC) concludes.

• Bushmen reveal ways of enduring in the Kalahari Desert on “Beyond Survival with Les Stroud” (9 p.m., Discovery).

• “Four Weddings” (9 p.m., TLC) enters its second season. Every episode of this eight-week series features four brides who attend and “judge” each other’s ceremonies on the basis of food, dress, venue, originality and overall experience. The winning bride gets a five-star dream honeymoon.

• Joan Rivers discusses the week’s fashion felonies and misdemeanors on “Fashion Police” (9 p.m., E!).

• Gene Tierney and Cornel Wilde star in the 1945 melodrama “Leave Her to Heaven” (9 p.m., TCM), a Technicolor gem as dark and twisted as any film noir shocker.

• The comedy showcase “Martin Lawrence Presents 1st Amendment Stand-up” (10 p.m., Starz) enters its fifth season.