If your image of Catholic nuns stopped somewhere between “The Singing Nun” and “The Sound of Music,” then you’re in for a surprise. The award-winning documentary “Sister Helen” (7 p.m., Cinemax) profiles the no-nonsense, occasionally foul-mouthed, Sinatra-obsessed Benedictine Sister Helen Travis, who founded a center in New York’s South Bronx for serious addicts.

The recovering heroin junkies and crack addicts live with Sister Helen in a rundown building supported only by the tenant’s rent and private charities. There, Sister Helen ladles out the tough love and discipline that her self-deceptive charges need to survive. But they also share meals, laughs and profound conversations about their lives, hopes and lost opportunities.

Unlike many nuns who enter a convent in their teens, Helen only became a nun at age 56, after the deaths of her husband and two sons. A former steady drinker, Helen lost one son to violence and another to drugs.

She saw her vocation as a chance at redemption. “I tried to do for other people’s sons what I didn’t do for my own,” she mournfully admits. “This house is a second chance to do it right.”

A funny, moving and unforgettable character study, “Sister Helen” would make a terrific television movie or big-screen motion picture. It just might be difficult to cast a contemporary actress who could capture Helen’s combination of ornery feistiness and big-hearted compassion.

• Jacqueline Bisset guest stars on the 100th episode of “Law & Order: Special Victims Unit” (9 p.m., NBC). And it’s a grim story, even for this series. Bisset plays the haughty antique store-owning mother of a former junkie linked to a particularly gruesome act of revenge.

While I admire the “Law & Order” franchise for packing plenty of twists and turns into each hourlong story, this installment seems rushed, and some of the performances almost over-the-top. This commemorative episode is not one of the series’ best.

Look for Mickey Hargitay (father of star Mariska Hargitay and one-time husband of Jayne Mansfield) in a cameo.

Tonight’s other highlights

• High drama at the tailgate party on “Gilmore Girls” (7 p.m., WB).
• Diahann Carol guest stars as Mavis’ bipolar mother on “Whoopi” (7 p.m., NBC)
• Is the Earth’s magnetic core shifting? Growing weaker? Some scientists think so, and they’re plenty worried in “Magnetic Storm” on “Nova” (7 p.m., PBS).
• Robert’s buddy (series producer Will Smith) takes a fancy to Neesee on “All of Us” (7:30 p.m., UPN)
• A school bus with 13 passengers vanishes on “Without a Trace” (8 p.m., CBS).
• Lillith returns on “Frasier” (8 p.m., NBC).
• Officials prepare for Salizar’s release on “24” (8 p.m., Fox).
• A tennis star is kidnapped on “CSI: Miami” (9 p.m., CBS).
• The serial killer comes to justice on “NYPD Blue” (9 p.m., ABC).
• “Queer Eye for the Straight Guy” (9 p.m., Bravo) returns with new makeovers.
• Sissy Spacek won the Best Actress Oscar for her portrayal of country singer Loretta Lynn in the 1980 screen biography “Coal Miner’s Daughter” (7 p.m., AMC).

Late night

Alec Baldwin appears on “Late Show with David Letterman” (10:35 p.m., CBS) … Jay Leno welcomes Billy Bob Thornton and Kid Rock on “The Tonight Show” (10:35 p.m., NBC).

William H. Macy, Heidi Klum and The Strokes are booked on “Late Night with Conan O’Brien” (11:35 p.m., NBC) … Dana Delany and Pauly Shore appear on “The Late, Late Show with Craig Kilborn” (11:37 p.m., CBS).