Carl makes his way through the bowels of the prison, following the sounds of the screams to right outside the boiler room. The memories of that place don't prevent him from efficiently head-shooting a walker that jumps out at him from nowhere, and he proceeds to the next area, where Tyrese and his group are in yet another pitched battle with marauding zombies. Carl shoots the last of the ones they're currently dealing with and yells, "Come on! Hurry!" They all move out, followed by walkers, dragging their bitten comrade, Donna, while Sasha and Carl fight a rearguard action. Carl says they have to leave her, but Tyrese refuses, and keeps schlepping her through the corridors in a fireman's carry, the better for her to take a chunk out of him when the mood strikes her.
Michonne is getting nothing from the Governor's front door, but after hearing some noises from behind the door to his creepy-ass den, she gets up and kicks that one open. There she finds the pyramid of fish-tank heads, and seems to recognize the helicopter pilot at its apex. Of course the source of the noise is behind the grate, so Michonne does what she does: opens it up and waits for whatever to come out. Seeing it's a little girl, she drops her sword in horror and tells Penny to come to her, apparently deaf to the soft zombie-snarls coming from under the head-bag. She undoes the chain at Penny's collar and pulls off the sack, which is of course when she first realizes what she's dealing with as Penny lunges at her. She grabs the girl by the back of the neck, turns her away, and raises her sword, and just then is when the Governor shows up in the doorway, screaming, "No!" with his gun leveled.
Michonne moves so that Penny's between herself and the Governor, and the Governor begs Michonne not to hurt her, holstering his gun and then laying aside the gunbelt. Penny is a surprisingly effective human-shaped shield considering she's three feet shorter than the target. Michonne watches the Governor in horror and confusion, but a moment later, the point of her sword comes bursting out of Penny's mouth. So much for the shield. Without bothering to pick up his weapon, the Governor rushes Michonne in blind rage (spoiler!) and disarms her, commencing a rather savage fight whose highlights include Michonne strangling the Governor with her scabbard and the Governor bashing her head against the wall before smashing it through the side of one of the fish tanks. When he tries to pull her free, she pulls down several of them with her, so there's water and biting heads all over the floor on top of everything else. She gets in an elbow and goes for her katana, but he grabs her before she can reach it, so instead she pulls loose a shard of tank -- cutting her own hand in the process -- and sticks it into his right eye, then goes the extra mile by breaking the point off inside. He lies there screaming and crying and Michonne raises her sword again to dispatch him, only to hear a second screamed "No!" from the doorway.