Tonight, Melissa Joan Hart sheds her comic-book image to take on a serious topic. The star of “Sabrina the Teenage Witch” hosts “Fighting For Our Future” (6 p.m., Lifetime), a documentary that focuses on the more than a quarter of a million American women under the age of 40 who suffer from breast cancer. Too often, these women and their doctors initially believe that they are “too young” for the disease and find themselves misdiagnosed. For this and other reasons, young breast-cancer victims suffer a higher mortality rate. Younger patients also contend with a host of psychological issues, as well as the side effects of chemotherapy, which can include infertility and the early onset of menopause.

Remember Newt Gingrich? The popular and controversial former speaker of the House of Representatives appears on “Missile Wars” on “Frontline” (9 p.m., PBS). Gingrich has been a longtime proponent of the kind of anti-ballistic missile defense that the Bush administration is now developing, at a cost that could exceed $200 billion. He believes that with states like North Korea and Iran developing their own missile technology, it is imperative for the United States to create systems to shoot them down.

Critics contend that not even the most complex and expensive systems can prevent terror attacks like the hijackers who perpetrated the Sept. 11 attacks. Physicist Richard Garwin questions the cost and effectiveness of the missile defense system. “It would be foolhardy,” he warns, “to base our security on a 21st-century Maginot line.”

“Liberia: America’s Stepchild” (9 p.m., PBS) examines the troubled history of the West African republic founded in the 19th century and populated by former American slaves. The new settlers, known as “Americo-Liberians,” named their capital Monrovia, after President James Monroe. Through the years, the settlers also perpetrated a caste system that marginalized the indigenous tribes, even engaging in forced-labor practices that closely resembled the slave trade. After more than a century of corruption and cronyism, non-Americo-Liberians seized power in 1980 with a bloody coup that has resulted in more than two decades of division and terror.

Tonight’s other highlights

Game Two of National League Championships (7 p.m., Fox).

A socialite uses an earthquake to cover up the murder of her husband on “Monk” (7 p.m., ABC).

Potential home buyers discover the sellers’ corpses waiting inside on “CSI” (8 p.m., CBS).

Jim discovers Dawn’s secret on “Push, Nevada” (8 p.m., ABC).

Scheduled on “Primetime” (9 p.m., ABC): an interview with a high school shooter; a bizarre murder and kidnapping case involving the patent to a new “rocket belt” flying machine.

Series notes

Competition and elimination on “Survivor: Thailand” (7 p.m., CBS) … Rachel wants to hire Ross’s old pediatrician on “Friends” (7 p.m., NBC)… Sissy attends charm school on “Family Affair” (7 p.m., WB).

A medical contest on “Scrubs” (7:30 p.m., NBC) … Joel tries to save his parent’s marriage on “Do Over” (7:30 p.m., WB)

A new romance distracts Grace from the baby-making scheme on “Will & Grace” (8 p.m., NBC) … Jake takes an awkward liking to Gavin on “Good Morning, Miami” (8:30 p.m., NBC) … Improvisation on “Jamie Kennedy Experiment” (8 p.m., WB) … Eugene Levy guest stars on “Off Centre” (8:30 p.m., WB) … Witnesses recall a mysterious mall kidnapping on “Without a Trace” (9 p.m., CBS) … Carter protests the hospital’s lack of security on “ER” (9 p.m., NBC).