“Prime Suspect: The Scent of Darkness” almost became the last installment of the saga focusing on the dogged London police Detective Superintendent Jane Tennison because an exhausted Helen Mirren wanted to pull the plug.

“In a way, the very success of `Prime Suspect’ was scaring me and I was verymuch of two minds while I was doing `The Scent of Darkness,’ ” says the actress of the two hours airing at 9 p.m. Sunday on PBS’s “Masterpiece Theatre.”

Mirren, an actress trained in the classics who takes herself and her profession very seriously, said that “The series and the character remained wonderful, but I didn’t want to get too associated with playing a policewoman the rest of my life.”

Hence, there is a pall hanging over “The Scent of Darkness” stemming from Jane Tennison’s strong sense of alienation as she tries to solve a series of bizarre copy-cat murders that seem like mirror images of the killings committed in “Prime Suspect’s” premiere episode five years ago. Suddenly suspended from her job in the middle of the investigation, her reaction soon threatens her seemingly comfortable relationship with a psychologist (Stuart Wilson).

Meanwhile, Mirren is back in London for three months to film the “final final” four hours of “Prime Suspect” to be aired during the 1996-97 season.

It was the articulate and opinionated actress’ social conscience that brought her back to the minimalist role. The finale’s theme is the lethal mix of guns and youth.

“Jane Tennison hasn’t changed much over the past five years, except that she is a bit more confident about herself,” Mirren explains. “But the world around her has evolved. The reason I wanted to do one more season is that, in reality, guns in the hands of young people has come on in a big way in England during this time period. It has already caused a great deal of destruction in the American society; now I’m horrified that it is happening in the land I grew up in.”

The London-born 50-year-old actress, who received her early training at the National Youth Theatre and the Royal Shakespeare Company, claims she is not worried about being “type cast” — a term she hates. “I’ve done lots of different roles since getting involved with `Prime Suspect,’ but there are fans who absolutely adore the show and think I really am just like Jane Tennison.”

“Well, I’m intelligent like her, but I’m not very tough,” said Mirren.

“I have a high profile in America because of the series and it’s a great advantage to be seen as a police person on those awkward occasions when I don’t have proper identification with me and I’m trying to cash a check,” she continues. “Viewers tend to think that Jane Tennison wouldn’t bounce a check because it would ruin her career.

“I daydream about being stopped sometime by a policeman for driving too fast and let go due to professional courtesy — `Carry on!’ ”

Neither is the self-assured and outspoken actress afraid of Hollywood’s infamous age discrimination towards performers over the age of 40 — particularly women — suspecting that a lot of people in that category aren’t very good actors anyway.

“And that doesn’t help exactly,” says Mirren. “They didn’t realize that they were being employed in the first place because of what they looked like.

“That applies as much to men as to women,” she continues. “There’s a huge amount of men who don’t work after thirty-odd, or when the buffed body turns to a bit of beer belly. I also suspect that a lot of actors, particularly those working enormous hours-a-day in films, simply don’t want to act any more because the work on multi-million dollar pictures is too intense and exhausting. Life is just too short for them to continue.”

Regarding competent actors, Mirren feels that the age discrimination hue and cry is simply a misconception created by the media. “It simply isn’t true because the audience has been changing for a number of years. It used to be that only the young went to the cinema, but now the Baby Boomers are here in great numbers and they want to continue to go to the cinema. They are not like their parents, or even those one generation removed. And they’re more interested in a film’s story, subject and characters than whether or not the cast is very young.”

Mirren, who was born Ilynea Lydia Mironoff to an English mother and a Russian father, was “lit by a fire at quite an early age, 13 or 14, in terms of acting. The spark that lit me was not film or television; it was my mother taking me to the theater. Specifically, Shakespeare. I wanted to be a great Shakespearean actress. In school, I loved doing Shakespeare. It was more exciting to me than Doris Day movies….incidentally, Doris Day is a greatly underestimated actress.”

She made her stage debut with the National Youth Theatre at the age of 19 and eventually worked her way through just about every one of the Bard’s works during the ’70s and the ’80s. On the big screen, she never turned down sexually provocative parts if she deemed the story strong. Hence, Mirren’s fans have seen every inch of her nude body in such motion pictures as “The Cook, the Thief, His Wife and Her Lover,” “White Nights” and “The Comfort of Strangers.”

A nonconformist in every sense of the word, Mirren has had a series of famous lovers, including Liam Neeson, but never “settled down” with a husband and children. For the past 10 years, the lightly tattooed actress has shared a six-acre estate in the Hollywood Hills with American film director Taylor Hackford (who directed her in “White Nights”) while maintaining a modest apartment in London and a small home in the South of France.

Although Hollywood is her home base, Mirren makes most of her movies and telefilms overseas, including “The Madness of King George” (1994), which earned her an Oscar nomination. She was in New York doing the play “A Month in the Country” at Academy Awards time last year.

“The thing that made me nervous about the whole thing was trying to decide what to wear,” she says. “The Oscars is so frightening and intimidating because nobody cares if you win or lose, they only care about what you wear. There should be an Oscar for `Best Dressed.’ ”

She has two films coming out later this year, “Losing Chase” (“It’s a sweet kind of gay movie”) for cable television and the feature “Somebody’s Son” (“I’m the mother of an IRA prisoner”), but concedes it probably will take some time before the Jane Tennison persona vaporizes.

“It has given me an aura in America, where people tend to identify you with your character. I think that’s because people tend to play themselves on American television, as in `Roseanne’ and `Ellen.’ “