Seo turns human.
She can still remember everything. Can still think of everything she once thought of — although she gets headaches from thinking of complex temporal stuff. But Seo has lost all connection to her ship, and she can no longer control her Key energies.
That means she has no hope of getting to Gallifrey, in its own past, so she can fix this.
D'Hoffryn gloats a little more.
And then the Vengeance Demons leave. To let Seo suffer in the ruined world she helped them create.
Seo decides that it doesn't matter if she's human or not. She has to find a way to stop this.
"There is a way," says Seo. "There has to be. Or the Moment wouldn't be here."
Dawn doesn't get it.
"D'Hoffryn said that he was able to control the Moment because he'd gained the Time Lords' secrets," Seo explains. "But that's a lie, because the Time Lords couldn't control the Moment, either. They were so scared of her, they locked her in a vault and forgot about her."
"So how...?" Dawn starts.
"If the Moment is here," Seo says, "it's because she wants to be. And if I know her at all... she's here because all this is somehow reversible. She's not really with the Vengeance Demons — she's trying to give us clues about how to reverse this!"
But before Seo can set out to stop this, Dawn first insists on checking on her children. Especially in a world as full of demons and chaos as this, Dawn has to make sure they're safe!
They head to Dawn's home, but it's empty.
The neighbors all remember Dawn's children, but only say that the children disappeared around the same time that Dawn did.
Seo remembers that Dawn's children had disappeared in the undestroyed version of Earth, too.
"Maybe," says Seo, "when the Victor took you and killed Keith… he killed your children, too."
Dawn spins on Seo, and shakes her. "Don't you dare say that!" Dawn shouts. "Don't you ever say that! They have to be safe! They have to be!"
But Seo insists that the only way to find Dawn's children will be to undo both the Time Lords' meddling, and the Vengeance Demons' reign.
"And that means, the first thing we need to do," says Seo, "is find Mom."
This proves to be extremely difficult.
They try to use the resources at A Charitable Earth, but it's been destroyed. Donna, Shaun, and Ace are all dead. It looks like they died trying to take down the Vengeance Demons, and when A Charitable Earth fell, Ace had blown up any alien tech they had left, to stop it getting into the wrong hands.
Seo and Dawn then try to find people they know at UNIT. But no luck there, either. All of their UNIT friends are either on the side of the Vengeance Demons, or they're dead.
Seo and Dawn then track down Gwen Cooper, and try to find Jack. But Gwen's never met anyone named Jack, and doesn't know anything about an institution called "Torchwood."
With a little digging, Dawn and Seo discover that Torchwood never existed.
Jack may never have existed, either. If he did, he certainly didn't travel through time, like the Jack they knew. So he'd be no help.
However, their digging does reveal one thing.
Willow still exists.
She's a high-ranking Vengeance Demon.
"If anyone knows where to find Buffy," says Dawn, "it'll be Willow. She'll know."
This whole time, Dawn has been trying to find her children. With no luck.
Seo and Dawn chase down Willow, only to discover that Willow is not happy to see them. And she's particularly upset to meet Seo.
"Yes, I know who you both are," Willow says, coldly. "The sister and daughter of that Buffy Summers. And if D'Hoffryn hadn't claimed the right to vengeance on you, himself, I'd have taken it for him."
Both Dawn and Seo are a little floored by this.
That's when they discover something astonishing.
"Buffy Summers made my life hell, the moment she moved to Sunnydale," says Willow. "A vapid, shallow person, who hung out with all the popular girls and took delight in making sure I was humiliated, every single day."
"That's not right," Dawn insists. "Buffy was your best friend."
Willow laughs, bitterly. "As if I'd befriend someone like her!"
Willow then says that she began exploring witchcraft because she'd heard a rumor that Buffy was something called a "Slayer". Buffy had decided she didn't care about being a Slayer, though, and had been shirking her Slayer duties ever since Hemory High School.
But Willow knew, the only way to take down a Slayer would be with witchcraft.
Willow started using magic to get her revenge on her most vicious high school bully, when D'Hoffryn appeared. D'Hoffryn warned Willow that interfering with Buffy's timeline would be a bad idea, "for she is a Line Hopper, Willow, and her fate is sealed. But of course… I can make you a better offer."
D'Hoffryn had offered her transformation into a vengeance demon. And give her a high-ranking position.
Willow had agreed.
"D'Hoffryn never explained what he meant by 'Line Hopper'," said Willow. "But I saw for myself what happened when Anyanka tried to change around Buffy's timeline. Apparently, whatever Buffy is, we Vengeance Demons can't touch her." Her voice dropped, low and edgy. "But we can touch the people around her."
She's glaring daggers at Seo, now.
And it's obvious that she heard about Seo from D'Hoffryn. And really wanted to tear Seo down and torture her, in retaliation for what Buffy had done in high school.
"You hate her that much?" Seo asks.
"She didn't care about people," said Willow. "Didn't care about the world! I saw her feed people she didn't like to demons, just to get herself out of a scrape. Whoever decided to make her the Slayer, they obviously had no taste."
Seo and Dawn are still trying to figure this whole thing out.
Because this is not right!
At all!
The Buffy they knew hadn't been cruel or vapid and shallow. Even though Willow insists that no one can alter Buffy, both Dawn and Seo know that someone already has.
"Actually," Willow adds, "I was surprised to hear that she jumped into that portal to save you, Dawn. I guess she did care about something, in the end."
Seo suddenly realizes something. If Buffy had jumped into that portal... but had been a completely different person, who wouldn't make one crucial wish at one crucial moment...
"The paradox," says Seo. "That's why they changed Mom. Because he wasn't in the right universe, when Gallifrey was destroyed; so he must have survived. And they didn't want that."
It takes Dawn a while to get it, but she does.
"They changed Buffy to be a certain way," Dawn clarifies, "to make sure that the Doctor never existed?!" She shakes her head. "How does that even work? I thought the Doctor was the reason Gallifrey survived that Time War in the first place."
Now Willow seems highly amused. "The Doctor?" she asks. "What do you want with him?"
This makes both Dawn and Seo do a double-take.
Neither of them had expected that, in a universe ruled by Vengeance Demons, that the Doctor would still be alive. Or that he'd even exist in the first place.
"He's still around?" Seo asks.
"Of course," says Willow, with a laugh. "He's been on Earth for the last 1,000 years." Then, still highly amused, "Why? Do you want to see him?"
They obviously both do.
Willow considers. "D'Hoffryn wouldn't like it," she admits. "But… for a chance to watch you suffer, Seo, when you see…"
Willow makes up her mind.
Yes, she'll let them see the Doctor.
She's sure that, if they care so much about him, and have so much hope that he can solve their problems, showing him to them will only serve to sate Willow's appetite for vengeance.