Although she’s currently starring as Queen Elizabeth II in Stephen Frears’s “The Queen,” Helen Mirren’s grandest and most durable role has been set not in tourist-ringed palaces and cloud-capped castles but in grubby police stations and sterile interrogation rooms. In “Prime Suspect,” as Detective Superintendent Jane Tennison, a lonely, stubborn, workaholic avenger of the victims of heinous crimes, she has often seemed not so much England’s queen as its conscience, a burden she has worn like a crown of thorns through seven separate four-hour episodes of British television’s most celebrated police procedural.

Only three years separate “Prime Suspect Six” and the arrival on PBS this Sunday of part one of “Prime Suspect Seven,” or as it’s officially known, “Prime Suspect: The Final Act.” (Part two will be shown on Sunday, November 19.) Yet those three years feel more like 10. (Tennison’s hair, ash-blonde in the last episode, is now silver.) It’s as if both she and her country have slipped into fast-forward, rushing (respectively) toward retirement and a feral multicultural future. “Prime Suspect” has rarely shown England in a flattering light, but there are moments in this concluding episode when Tennison seems so impatient, angry, and even bored by its teeming anarchy that she can barely bring herself to listen to a suspect’s confession, let alone end her final case and career on a high note.

There’s also the matter of her alcoholism, apparently now a full-blown addiction, which surely can be read as a reflection not only of her personal demons — emotional isolation, a life buried under a career — but of a barely articulated national despair: Why is there so much violence and crime? Still, did we really need to see one of television’s greatest sleuths at an AA meeting announcing, “My name’s Jane Tennison, and I’m an alcoholic?” Surely a statement such as, “My name’s Jane Tennison, and this bloody country looks like it’s beyond saving” would have been more appropriate.

When Tennison appeared in the first “Prime Suspect” in 1991, the police force was portrayed as almost entirely white, male, and hostile to feminine or ethnic intrusion. Now young women are everywhere on the force, as are minorities in general. One can’t help noticing the irony that Tennison’s attitude toward younger policewomen tends to be prickly. Or that her only confidante is Bill Otley (Tom Bell), a wily, old-school veteran of two earlier episodes, a cop who was never happy taking orders from a woman.

Tennison runs into Otley (though first she tries to run away — always her first instinct) at an AA meeting. They haven’t seen each other in 10 years. Once her enemy, this broken, retired detective more or less instantly becomes her confidante. After all, he’s as alienated as she is, and there are no other applicants for the position. Tennison, with her stiff upper lip and emotional rectitude, has become a “dinosaur.” No one on the force knows how to talk to her anymore. They can only look at this legend with a kind of awe and a vague desire that she’d go away. In fact, even though she’s retiring in weeks, they try to push her out early.

The plot of “Prime Suspect: The Final Act” centers on the disappearance of a 14-year-old London schoolgirl — familiar territory for the series, which has often concerned itself with pedophilia and child abuse. The girl, Sallie, has unusually strict parents. Her bulldog of a dad, Tony Sturdy (Gary Lewis) is particularly anxious to protect her from the city’s cultural and sexual mayhem and believes he’s succeeded. Of course, he hasn’t.

The most likely key to deciphering what became of Sallie is her best friend, Penny (Laura Greenwood), the precocious daughter (also 14) of their school’s headmaster. A rebel with artistic leanings, Penny reminds Tennison of her younger self. When Sallie is found dead and two months pregnant, Penny becomes an even more important witness to whom Tennison forms a strong emotional attachment. She lends Penny books and takes her to a museum to view a portrait of a young girl by Sir Joshua Reynolds; but she also terrifies the teenager when she takes her for a drive and nearly ploughs into another car because she’s drunk.

In every “Prime Suspect” there’s a moment when Tennison orders an underling to dig up the dirt on a suspect. It happens in this episode, too, but for the first time in the series it’s Tennison’s own life that’s brought to the fore. As you’d expect, her biographical details are rather sparse. Her father has been dying for some time, but Tennison finds out only a week or two before the end. “Why didn’t you tell me?” she demands of her conventional, married sister. (Did we even know she had one?) Answer: “You’re always so busy!” And so she is. Tennison has been so busy for so long that the rest of life, even what’s left of her family, has passed her by. Why bother calling her? She’s always “on a case.”

Final acts are hard to pull off, but this one is handled admirably. Looking back on the series as a whole, a complex portrait of a driven single professional woman emerges, if one that’s mostly (up to now) been glimpsed between the lines: some brief affairs, an abortion, medical visits, a father who was one of the first to liberate Belsen and hoped to shield his daughter from the evil of the world. (Instead, she jumped right into it.) And work, work, work. The impression is of a long novel with half the chapters missing.

At the risk of sounding ungrateful, one wishes the writers could have come up with more than seven editions of “Prime Suspect” in the 15 years since its inception. Ms. Mirren has been quoted as saying it was time to end the series, and she certainly does a memorable job here of playing a groundbreaking feminist crime-fighter holding on to her pride as she’s being put out to pasture. (Having triumphed over sexism, she succumbs to ageism — or enforced retirement.) It’s a measure of the hold she exerts over our imaginations that one wonders, even as she coolly skips her farewell party and disappears down a London street, presumably forever, what exactly she’ll do with her remaining years.

bbernhard@nysun.com