PBS embarked Thursday night on one of television’s seamiest plunges into the raw, slimy underbelly of society.

But, hard as it is to watch, the powerful four-hour British miniseries “Prime Suspect 3” is award-worthy story telling, a viewing event of the season.It’s a fitting welcome back for Helen Mirren as Detective Chief Inspector Jane Tennison, one of the most complex and compelling female roles to air on American television. This third story in the “Prime Suspect” series is part of PBS’s “Mystery!” package. It will air on subsequent Thursdays on most PBS stations. It opened with Tennison transferred from homicide to a London vice squad, where she was handed the reins on a murder case that soon had the screen swirling with rich and powerful pedophiles, “rent boy” teen prostitutes, predator procurers and tranvestites.

It is a disturbing story that oozes pain and fear. Some British critics call it “deliberately shocking.” Writer Lynda La Plante says that is what she intended, that she wrote from anger “at a society that leaves children to live on the streets.” Mirren agrees that it must be told because child prostitution and abuse are often ignored. “It needs to be looked at because it’s out there amongst us,” Mirren told TV critics. “It’s out there right now…. We all like to pretend it’s not there . . . but the fact is, there is a huge industry that’s based on child prostitution. . . . This particular piece puts the searchlight on that corruption.” La Plante wrote the hugely praised first “Prime Suspect” in which Tennison battled sexism within the force while pursuing a serial killer. For the second miniseries, which focused on racism inside and outside the department, La Plante supplied only the general story line.

Her return as the script writer serves up a riveting successor to “Prime Suspect 2,” which won a prime-time Emmy as last season’s best miniseries. La Plante said she based “Prime Suspect 3” on the real case of a convicted pedophile who served five years in jail and was released, without psychiatric treatment, to roam the streets again. “Until I started researching this, I never realized what a dirty, tragic underworld it is,” she said of her time spent around London’s Waterloo Station, an area rife with child prostitution. Added Mirren: “The really scary thing is that it isn’t made up. Most of the content can be found in TV documentaries. We haven’t needed to exaggerate for the sake of drama.”

To represent one of the worst sides of this life, La Plante created the character of Jimmy Jackson, a procurer who entraps and tortures young boys and girls wandering the city’s streets. As Jackson, David Thewlis delivers a memorable performance, exuding chilling confidence from the edge of the soulless abyss behind his scruffy haircut and sparse chin whiskers. The 30-year-old British actor made a living over the past decade playing mostly sensitive, tender characters. This is only his second bad-guy role. Granada TV, which produced “Prime Suspect 3,” cast real transvestites rather than dressed-up actors for the background roles. “We did a day’s filming in a drag club with drag queens,” said Thewlis. “It was interesting to watch this very heterosexual male English crew confronted with real drag queens and how their reactions changed from one of total derision at the beginning of the day to a kind of nice acceptance at the end.”

But this is Tennison’s tale, and Mirren is dead-on again with her interpretation of the middle-aged, demanding workaholic boss whose professional and private lives are as loose-ended as most real lives. “I see her as driven, obsessive, vulnerable, unpleasantly egotistical and confused, like we all are,” Mirren said. The portrayal of Tennison doing her job despite constant sexist barriers has brought Mirren positive responses from women in many types of jobs, not just police work. “The thing they like about this piece is that it’s political without being propagandist,” she said. To a critic who suggested Tennison is continually unhappy, Mirren disagreed: “I don’t see her as unhappy. No, on the contrary, I see her as absolutely participating in life. She’s out there. She’s in life. And it’s so rare, as a woman, that you get to play those roles.”

Mirren, 48, with her blond hair cropped shorter, again plays Tennison with a minimum of makeup and a maximum of “bad day” looks that everyone, except the common characters on TV, must endure. “I always want to look good,” Mirren said. “I’m always saying to the cameraman, `Put the camera off a bit, I don’t look good like that.’ But on the other hand, the great thing about this character is you can let go of all that . . . because that’s not the primary importance here, and that’s a great relief for a woman.” A year ago Mirren said she would do “Prime Suspect 3” and call it quits, but that has changed. She and the folks at “Mystery!” report that “Prime Suspect 4” is in the works, though the format may change, possibly three fully contained two-hour productions that have an underlying story as a connecting tie. There is no further word on last year’s report that a theatrical version of “Prime Suspect” is being considered. For now there is no need, because we have the exceptional “Prime Suspect 3.” It’s sure to be “appointment” viewing for fans of the series, some of whom may have to actually figure out their VCRs, so they won’t miss new “sweeps” episodes of the popular sitcoms “Seinfeld” and “Frasier” that play opposite on NBC.