Magnificent ending (spoilers)
Dame Mirren's performance in the final scenes of Prime Suspect 7 floored me.
After Penny is brought in and demanding to see DSI Tennison, we see Tennison's steely resolve not to see her, but we also see the hurt from being betrayed and the self-disgust at having been duped.
When Tennison finally does enter the interrogation room, she remains aloof and even more reserved than usual, but as Penny slowly unravels and the evidence against her proves inescapable, Tennison's compassion slowly returns for the young teen she befriended and once loved.
Finally, when Penny realizes all is lost, she turns on Tennison, but at this point, Tennison's anger has dissolved and all she can do is to respond that she never lied to Penny. Then, as Tennison seems to accept the hard truth, she has a flash which seems to offer a way out for Penny, and this brief, tenuous hope lifts Tennison slightly, who offers that Penny only meant to kill the fetus, and not victim herself. Penny refutes this, and Tennison collapses into herself.
The other detectives come in and take Penny away, but Tennison remains head down, unable to face anyone. Then, in a broken voice, made all the more powerful because she has always projected strength and hidden her emotions in public, she says that they got what they wanted. But she also got what she wanted, Sallie's murderer, as she promised Sallie's father. At that moment, however, she was not being a detective, but a lonely, driven alcoholic at the end of her career who was forced to cut her own lifeline to a world of hope and possibilities.
We do not end on this dark note, of course, as Sallie's parents thank Tennison and remind her of her years of service to all the victim's families, but the tragedy of the interview scene and the way that Dame Mirren played it mark, for me, the pinnacle of the series and the career of DSI Tennison: a determined, passionate individual who pursued her profession without apology or seeming regret, only to be reminded, at the very end, of what she sacrificed in the process. I would not call it a Pyrrhic victory, but it is a victory nonetheless.