Rose comes around on the scary TARDIS action, and she gasps and flinches and wigs and runs toward it. Inside, Captain Jack and the Doctor work quickly and furiously to unplug the extrapothingy. Jack yells, "I've disconnected it but it's still feeding off the engine! It's using the TARDIS -- I can't stop it!" The Doctor verifies that Cardiff is not the only issue here; it's going to rip apart the planet. Rose comes running in, all freaked out, and Blon giggles. She rips off her human arm like the swan boy in the fairy tale, and grabs Rose with a big Slitheen claw: "One wrong move and she snaps like a promise." She always did have a way with language, didn't she? The Doctor's like, "Still a snake no matter what pocket," and sighs, and Blon tells him to shut it: "I've had you bleating all night, poor baby." She calls out to Captain Jack -- "Flyboy!" -- to drop the extrapolator at her feet, the better to surf off with. After ten seconds of Rose-strangling, the Doctor gives Jack the go-ahead, and Blon very pleasantly thanks them for playing perfectly into her plan. Rose asks an Excellent Question, so Blon dutifully explains that, in the event of getting waylaid, she planned to use the presumably advanced tech of whoever could stop her, and programmed the extrapolator to "go to Plan B." I like any villain with a Plan B, but especially Margaret Blaine. She's so cool. She yanks on Rose's hair and says that Plan B meant it would "lock onto the nearest alien power source and open the Rift." Which would be a strikingly contingent plan, if it weren't for the fact that her captors would have to have pretty good stuff, like she said. Jack complains that the whole planet's going to be destroyed, and Blon's like, "Duh, and also you." She steps up onto the extrapolator, one hand around Rose's neck. "...While I ride this board over the crest of the inferno, all the way to freedom. Stand back, boys! Surf's up." And outside, there is lightning, because it's all very dramatic and suspenseful.
Then a thing happens. Either a very cheesy thing, or the only possible thing, depending on where you're standing. A panel in the TARDIS wall opens up, bathing them in beautiful and blinding white light. "Of course," the Doctor smiles from his console. "Opening the rift means you'll pull this ship apart." She's like, "Suck it, tool," but he's not done: "It's not just any old power source, it's the TARDIS. My TARDIS. The best ship in the universe." Blon doesn't know about these speeches, because the last time he won, she was being blown up, so she just snots off at him. Rose asks what the light's about. "The heart of the TARDIS. This ship's alive. You've opened its soul." And Blon stares into the light, and whispers, entranced. "It's so bright...beautiful..." It's the heart of the thing that connects you to the infinite. Of course it's bright. It's the most beautiful thing in the world. And the Doctor looks on: "Look at it, Blon. Look inside, Blon Fel Fotch. Look at the light." She stares, lets Rose go. Rose stumbles over to Jack, as a beautiful, blissful smile breaks out across Blon's face. She had to stay human for this, had to keep a human arm, or we wouldn't get it. She's our agent in this. She looks up into the Doctor's smiling, wonderful face. And she thanks him. Genuinely, beautifully, gorgeously thanks him. And she's gone.
The Doctor jumps around the console and cautions his crew to close their eyes, slamming levers and pushing buttons. The panel closes. The light goes away. The Doctor, Jack, and Rose work, all three in tandem, to shut down the TARDIS. Don't let the unutterable cause you to forget there's a Rift mid-opening, or else you might as well join a cult. The family gets it done, the Doctor thanks them. In the silence, Rose asks what happened to "Margaret." Captain Jack thinks that she was burnt up, a self-execution, but the Doctor knows better. He looks down into her discarded skin-suit, that pile of lies, as Rose asks where she went. The Doctor answers, "She looked into the heart of the TARDIS, and even I don't know how strong that is. And the ship's telepathic -- like I told you, Rose. Gets inside your head. Translates alien languages. Maybe the raw energy can translate all sorts of thoughts..." Maybe it can grant wishes. Maybe it can grant choices. He kneels down to the skin-suit, Rose and Jack huddled behind him, and the Doctor pulls out an egg: "Here she is!" Rose: "She's an egg?" They both are. Grace isn't the end, it's the beginning.