The Cybermen notice somebody getting close, and Mercy's like, "What? Check it out! So they Voltron the head around to look at him, and his hair going absolutely crazy in the wind. He aims the infostamps at her, and she's like, "Another man, come to assert himself against me in the night!" Which would be so, so compelling if any of this made sense. So the Doctor levels with her -- "You might have the most remarkable mind this world has ever seen, strong enough to control the Cybermen themselves" -- and she tells him to shove it, because she doesn't need his permission to be awesome, but he tells her she kind of does, because she's stomping on London and is going to die if she doesn't calm down. He offers to use the Dalek thingy from the other Dalek thingy to send her somewhere else where there aren't any people to Cyberconvert.
"I have the world below," Mercy points out. "And it is abundant with so many minds, ready to become extensions of me. Why would I leave this place?" Because he's going to stop her. "What do you make of me, sir? An idiot?" No, but that's not the question, the question is whether she's going to make him into the kind of a guy who blows up awesome robot-controlling ladies. She is firmly in the affirmative on that one, and he's sad. "You make me into this," he says, and fires vibes into the robot's mouth where she's hanging out. And nothing happens, so she laughs at him, but it's too hard to even care because A) this shit fell apart a good while ago and B) his hair is now Edward Cullen amounts of crazy, and it's really distracting.
"I wasn't trying to kill you. All I did was break the Cyber-connection, leaving your mind open." She begins to wig out, because of that goddamn human spirit thing no doubt. "Open, I think, for the first time in far too many years. So you can see. Just look at yourself. Look at what you've done." She starts hyperventilating like a hooker that didn't know her place and ended up turning into a giant robot. He apologizes, and she realizes what a dick she's been acting like, and then screams so loud that her Cybermen explode, and then she explodes, and then it's just a giant robot with nobody driving it, and it's about to fall on London and kill everybody, but then the Dalek thingy does something or another, and the robot disappears. The Doctor is sad, because that was dumb.
And the sad thing is, RTD and Gardner realizes later on that obviously Miss Hartigan -- in addition to making any sense whatsoever -- should have been the one to disappear the CyberKing somehow, herself, without any reference to the stupid Dalek thingies at all. That would have been great. Instead, we just have the Doctor giving this "I'm killing you for your own good" speech he's already given a million times, and a bad guy who was neither bad nor a guy, but somehow and for some dumb reason became both.