So, apparently there was a movie with Sarah Michelle Gellar as Buffy Summers, where Buffy was recruited/drafted by the military between S4 & S5 (I'm guessing -- it was vague, which I'll explain in a moment, but it would make sense here) in which she was sent under cover.
Her assignment is to hunt down Helen Mirren as a spy/assassin who'd gone rogue and is hunting down old spies/Cold War agents. Helen's shown scene included her murdering an old man in a move theater with a knife to the neck at the collar bone and stuffing the wound with straw so that the blood wouldn't be immediately noticed. Yeah, straw... in a crowded movie theater....
Helen Mirren's character was related to Gwyneth Paltrow, who played herself, but was also an analyst for the CIA/Pentagon and was trying to stop her grandmother without her ending up dead.
Buffy's short scene included was her in an enemy base/intelligence agency, wedged above some men's heads on a ceiling as they walk by, and then flipping silently down and over the camera, in this fancy somersault thingie over the camera lens and then rushing down a hallway with the camera at floor level watching her back as she raced away from us.
And, the reason these scenes were so short in my dream of this movie, was because it was being presented as a film review by Spoony... which, I have no idea, since his reviews are not ones that I regularly watch. Apparently, he is just a memorable dude.
Anyway, Spoony was the one to vaguely explain that this movie took place between two Buffy tv show seasons, without specifying which, though he did say that this is explained away in a throwaway line of dialog within the show during the latest season after the movie was released. His review suggested that he was looking-back at the movie, but how long ago in the past it took place wasn't clarified in my dream.
But, his review ended by showing another scene in which Dawn suddenly appears in the top brass' office as a general dude is discussing with a special forces type soldier-agent about finding Buffy to determine if she needed saving, or if she had turned rogue on them and had to be eliminated.
Dawn is with Illyria and Connor and they're all posing dramatically. Dawn gets a swooping zoom lens, where she says: "Where in the hell is my sister?!"
And then I snapped awake. I, alas, never got to see/hear the title of this fabulous movie about Buffy hunting down an elderly rogue agent killing other retired agents from the Cold War under pressure from some government agency.