Seo and Dawn do, eventually, confront the Moment. Seo demands to know why the Moment did all of this. Why did she make Seo destroy the world and the universe?!

"You told me... or will tell me... that the Time Lords should have been wiped out, long ago, in vengeance for what they did to your aunt," says the Moment. "But when you told me that, the Doctor had died on Trenzalore. Now, things have changed. Timelines have altered. With the Doctor alive, the Time Lords were suddenly eager to cover the whole thing up. And I was given the chance to show you what would happen... if you'd gotten your wish, and the Time Lords were all dead."

"But I wasn't really dead!" Dawn protests. "I was a stone segment!"

The Moment looks at Dawn. Very sadly.

"You were dead," the Moment tells Dawn. "Really dead. I'm sorry."

"How?" Dawn demands.

"Your energies saved the Time Lords," the Moment tells Dawn. "But the energies were so powerful, that they ripped through the crystal lattice. Your segment shattered into a million pieces. Gallifrey was only saved... could only be saved... because you died."

Dawn goes white at this.

"No," Seo cuts in. "It's because I never showed up. I could have controlled those energies. I could have stopped this!" She gets even more angry. "Why didn't they let me know?! Why didn't anyone tell me?!"

"Some wanted you to know, others did not," says the Moment. "If you'd turned up to save the day... it would have made you and Jenny look like heroes. Not everyone on Gallifrey wanted that."

"They'd have sacrificed Dawn, just to score a political point?!" Seo cries.

"No. But they would to wipe out her family's future."

Seo gets absolutely furious. She starts saying that maybe the Time Lords deserve to be dead, even though she could now see what harm such a thing would do. Maybe instead of saving the Time Lords, she should just go back and do a more thorough job of wiping them out!

"And that's why I did this," said the Moment. "Showed you this. You never forgave them for killing your aunt, Seo. Especially not when you learned about her children. But they never forgave you, either — for what you did, next."

Now we get to see what happened in the timeline in which the Doctor died on Trenzalore.

In that timeline, things begin basically the same.

Dawn is captured, her husband killed. She winds up on Gallifrey, escapes, and goes off to find Jenny.

Everything happens just as it did for Dawn, this time around.

Except... at the end.

Instead of walking into her own death, willingly, Dawn put up a fight to the bitter end. Jenny reassured her that the Doctor knew how to convert segments back into people, and Dawn shouted, "You think he can still do that from beyond the grave?!"

At another Time Lord's provocation (Weryn), Jenny wound up converting Dawn into a segment through force and trickery. Dawn screaming the whole time to for Jenny to please find another way!

Then Jenny activated the machine.

Promising Dawn that, "I'll find a way to get you back. I swear."

But when it was over, Jenny looked through the smoke. Horrified. As she raced out and tried to gather the shattered pieces of Dawn's broken crystal segment, unable to believe what she was seeing.

"Jenny," said Weryn. "What have you done?"

Jenny looks up at him.

"And right before the President resigns!" Weryn shook his head. "How do you expect to be President of Gallifrey, now? After the people discover you've murdered Dawn Summers-Peters, the human who saved our lives?"

Jenny realizes what he's talking about. The Matrix would hold the evidence showing that Jenny had forced Dawn to her death.

Weryn had given her the idea. But only because he'd known what would happen, next.

"You wanted this to happen!" Jenny accused. "So I'd look bad!"

Weryn shakes his head. "The evidence doesn't lie, Jenny."

Jenny scrambles to collect all the fractured fragments. "I'm not letting her die just because of some Gallifreyan politicians, vying for political power!" Jenny insists. "I'm finding a way to get her back, Weryn. And you can't stop me."

Jenny tried everything.

Even went back in time, to Earth, to briefly interact with the Monks of the Order of Dagon. And learn their secrets.

But it was ultimately futile.

Jenny learned that Dawn had died so that Gallifrey could live. To undo that would mean the fall of the Time Lords — if not now, then a long time from now. When Dawn's descendent brought them to their knees.

Then Seo discovered the truth.

Now, cut to Seo, in the timeline where the Doctor died on Trenzalore. She was so furious about Dawn's death, she returned to Gallifrey and attempted to destroy it.

"Father rewrote time," Seo said to the Time Lords, "but that was wrong. You should have stayed dead. You deserve to be dead. All of you!"

Seo explained that she was going to destroy Gallifrey because she had now learned that Time Lords should never have this kind of power over anything. She wanted to stop them from doing this to anyone else!

(But there's definitely a little bit of vengeance in the action, too.)

Jenny raced up to Seo, before she could destroy the world, though. She talked to Seo, and managed to convince her not to destroy it — by saying, "I'm not leaving. Kill them, and you kill me, Seo. And we both know you won't do that."

"I will if it means they never do this again!" Seo insisted.

Jenny managed to negotiate between Seo and the Time Lords, though. Jenny negotiated the lives of Dawn's children — all of them — to be restored to normal, in exchange for Seo letting the planet go.

The Time Lords agree because they figure they can find some other way to get rid of Dawn's descendants.

And Seo agrees because she knows she wouldn't actually kill Jenny. She doesn't really have the guts to destroy a whole planet. And she really does want Dawn's children back.

The moment Seo surrenders, the Time Lords attempt to execute her. Jenny intervenes and smuggles Seo off the planet, warning Seo never to return.

Seo replies by telling Jenny not to worry. She'd be very happy never to set eyes on Gallifrey again.

"But if they ever try anything like this again," Seo warns, "they're all dead. Even if I have to kill both you and myself to do it."

Back to the vengeance demon timeline. In which the Doctor survives past Trenzalore, and the vengeance demons have taken over the Earth and the universe.

Seo is now starting to understand why the Moment gave the vengeance demons this kind of power.

"You gave me the vengeance I wanted," Seo says. "And the ability to see its results."

And, of course, she gave Seo back Dawn. So that Dawn could make her choice, now that she really knew what was at stake.

Dawn gets it, suddenly.

"That's why the vengeance demons didn't kill me, when I tried to go against them," Dawn realizes. "It's why they haven't even tried to hurt me! Because their power wasn't in that crystal Seo held."

Yes.

Their power is in Dawn.

Because Seo wished that Dawn was still alive. So, while Dawn's still alive, all this can continue, and the change the vengeance demons made to the timeline will become permanent. The Vengeance Demons had been planning to make sure Dawn stayed alive forever, but they'd been distracted by the search for her kids.

And the Moment had made sure they stayed distracted.

"My life means the death of the Time Lords," Dawn says. They're on the top of a high-rise building, and Dawn heads to the edge. "My death means the destruction of the vengeance demons."

Seo tries to stop her. Offers to give herself up in Dawn's place.

But Dawn says no.

"It wouldn't work, Seo," says Dawn. "No jumping-into-a-portal-to-save-me, this time. I'm the one who was on Gallifrey, when it needed saving. I'm the one you wished to be alive — causing all this. There's only one person who can stop this... and it's me."

"Seo is fully human, now, anyways," the Moment adds. "Her key energies are uncontrollable. It wouldn't work."

"But Seo will go back to normal, if I jump, right?" Dawn asks. "Everything will go back to normal. Buffy. The Doctor. Seo. All those Time Lords."

"Yes."

Seo begs Dawn not to do this. She'll find another way! There has to be some way to save the universe and Dawn, at the same time.

"I'm not giving up my life for the universe," Dawn says. "Don't you get it, Seo? I'm giving it up for my children."

Seo doesn't understand.

"The Doctor's alive," said Dawn. "And because of that... Weryn's plan failed. Remember that."

Then she jumps.

And time gets reset back to where it was, before.

Seo finds herself back on Gallifrey.

A piece of crystal in her hand.

And Jenny is next to her, warning her, "Don't do anything stupid, Seo. I know you're upset — but the Time Lords aren't monsters! You don't know what things would be like if they were gone!"

Seo sighs.

And smashes the crystal underfoot.

"Yes, I do," Seo says. "I hate it. But I do." She turns to the door. "Now let's break out of here and find Weryn."