So Katherine and Aunt Lily buried Dylan in the woods, because Aunt Lily knew Wayne would make Dylan's death look like Katherine's fault. Katherine went on the run again, but first she picked up a Dylan lookalike from a Romanian orphanage. This new girl's father had been murdered, which explains the "your father was murdered" scrap of Aunt Lily's note that Dylan found earlier this season. And it also explains the deep scar in the hardwood under the rug in the old upstairs bedroom.
Which Katherine is showing Bree and Wayne, back in the present, as she says she's never told anyone this.
Down in the street, Adam pulls his stolen car up to the barricade blocking off the street for the party. "So you let her die," Wayne growls at Katherine, quietly furious. Adam drives right through the barricades and up onto Katherine's lawn, seconds before Wayne is about to shoot Katherine with her own gun. Hearing Adam burst into the house, Wayne leaves the women alone, and Bree starts untying Katherine. They hear crashes from below, and wait in suspense to see who's going to come through the door. It's Adam, although he looks like he was taken apart and then put back together by Carlos. The three of them go back downstairs, where Wayne is lying on the floor, hurt but alive and conscious. Adam leaves Katherine in charge of Wayne -- with the gun and everything -- while he and Bree go to call the cops. From the floor, stupid Wayne tells Katherine that his friends on the force will cover for him, and he'll come back and make Katherine pay. Because when you're helpless and someone's pointing a gun at you, what reason would you have to suddenly come over all agreeable? "I know you will," Katherine says, and shoots him dead. She drops the smoking gun on he floor, Adam takes her in his arms, and Bree goes out through the front door. Seeing her, Susan and Lynette come running. "We need to talk," Bree tells them urgently.
Gabby's the last of the four to arrive, and Susan catches her to bring her up to speed. Cut to a uniform trying unsuccessfully to get a handcuffed Katherine to talk, but she's just staring blankly ahead of herself. Outside, Bree is telling another cop that Katherine just managed to grab Wayne's arm before he killed them all, Susan is telling another officer about Katherine's backstory; Gabby is lying to still another about Katherine always telling them about how terrified she was; and Lynette says to a plainclothes detective that they all knew Wayne would pull something like this one day. So after Mrs. McC pointed the cops in the right direction, they still didn't get there until they had time to get their stories straight. I've just realized that Fairview is where cops go when they're too stupid to work in Sunnydale. Lynette's target, by the way, is that same detective who was buddies with Wayne, and he goes into the house to say this was obviously self-defense and Katherine needs to be uncuffed and brought to the hospital. Katherine isn't exactly flooded with gratitude. At least not for him. But as she's walked out to the ambulance, she's obviously grateful to the four ladies who just covered her ass. Dylan comes up, and they apologize to each other, and hug.