We first saw hints of this next story in Season 3, when the Tenth Seo told Dawn about her own death — and made it pretty clear that Dawn hadn't died well. Seo clearly blamed herself for it, and did things she wasn't proud of, because of it (and some things she was proud of, like adopting Chiara).

That, of course, was in the timeline in which the Doctor died on Trenzalore.

How will that change, now that the Doctor is alive?

What happened in that other timeline? What happens in this one? Does Dawn die an honorable death, or is she horribly murdered while she's pulled out of her own time?

This story answers all those questions.

I didn't ever get a chance to write this story, but I liked the summary I wrote for it, quite a lot. I'll caution you that it's a little complicated, and involves a lot of different timelines and storylines and alternate realities. But I do like it, quite a lot.

Of all the stories I wish I'd written, this one's the one I wish I'd written, most.

Here it goes.

(Dawn Summers-Peters, by the way, is Dawn's name after she gets married. Her husband is Keith Peters. Over many generations, the name "Summers-Peters" will eventually morph into a mixture of the two names: "Sompters".)

The Death of Dawn Summers-Peters

Seo materializes Oliver on Earth, and steps outside.

Then jumps about a foot in the air, as she hears a voice just beside her.

"So much suffering and so much loss," says the voice. "Your pain is like a living thing, squirming its way through the dimensional barriers."

Seo spins around, and notices a vengeance demon, beside her.

"We can offer you a way to deal with that pain," says another vengeance demon, behind her.

Seo spins around, again.

"That loss," says a third.

Seo turns to him.

"Your anger," says a fourth.

"Your need for justice," says a fifth.

"Your need for vengeance," says a sixth.

"Your need to stop your own guilt," says a seventh.

A whole group of vengeance demons flock around Seo, until she's spinning around in a circle so fast that she gets dizzy and nearly falls over.

Funny thing is… Seo doesn't feel vengeful. Or angry. Or in pain.

She mostly just feels confused.

"Sorry, why are you lot trying to make me swear vengeance?" Seo asks them. "I'm actually quite happy, at the moment. I beat back a load of alien invaders on a backwater planet in Andromeda, and I'm rather proud of it."

"A forgotten loss will sting even more," says a vengeance demon, "when it's finally remembered."

Seo has no idea what this means. And doesn't really care.

"Look, I'm not particularly sympathetic to you vengeance demons in the first place," Seo says, pushing past them. "So if you don't push off, right away — I'll get rid of you lot, same way I got rid of D'Hoffryn."

Then she heads away, to find her friends, Ace and Donna.

When Seo arrives at A Charitable Earth, Ace looks both relieved and grateful.

"It's Donna," Ace explains, leading Seo into the back. "She's gone mad. And not in a brain-melting-down sort of way, either!"

When Seo arrives in the back, she finds Donna, there.

Everyone else has grown older over the many years since they first set up A Charitable Earth, but Donna's aged much slower than the others. She's still red-headed, loud-mouthed Donna — and at the moment, she's terrifying everyone in sight.

"Oi! You lot can't just forget a whole family, here!" Donna shouts. She grabs up one of the secretaries at A Charitable Earth. "I'm talking to you, sunshine! So you better tell me… where's Dawn Summers-Peters? Where's her husband, Keith? And where are her three children?!"

Seo frowns.

Dawn… Summers-Peters?

Seo knows that her real name is Dawn Summers, of course. Although how Donna knows that is a mystery — probably a leaked Doctor-memory, Seo can only assume. But who's Dawn Summers-Peters? And what husband and kids are Donna talking about?

"You remembered them all, yesterday!" Donna insists. "Then, three hours ago, something changed. She was gone, and you lot started saying she'd died years ago. And now… you don't remember any of them ever existed!"

"Told you," Ace mutters to Seo. "She's lost it. Keeps thinking there's someone we're forgetting, who used to work here."

Donna hears.

Turns on Ace.

"What? You think I'm making this up?" Donna charges towards Ace, shoving a finger in her face. "Listen, mate! I may have memory problems, but I don't remember things that aren't there. A whole family can't just disappear like they'd never existed in the first place! It's rubbish!"

Seo suddenly gets a deep chill.

Memory.

And someone named… Dawn…

"You remember this person?" Seo asks Donna. "Even though the rest of us don't?"

"Of course I bloomin' do!" Donna insists. "She was working here until yesterday!"

Seo thinks about all the vengeance demons outside, who insisted that Seo had forgotten some kind of horrible loss. She thinks about the story of her own life, about the Key popping into existence. And if Seo is an alternate-timeline of the battle between the Monks of the Order of Dagon and Glory…

Then what if there was another Dawn, in this timeline?

Someone the Monks conjured up, by altering everyone's memories?

And what if… someone out there… found a way to undo that magic, after they killed this other-Dawn? So that no one could remember she'd ever existed?

"I have to go," Seo says.

She runs out of the building. Back to the vengeance demons.

"You know something about this," Seo tells them all, running up to them, out of breath. She pauses, panting, struggling to get the words out as quickly as she can in between pants. "You know what I forgot."

"Dawn Summers-Peters," a vengeance demon says.

"One of the most important people in your life," another agrees. "Your family."

"And someone I'd swear vengeance over, if I remembered her," Seo cuts in, sharply. "But I don't remember her! I can't remember anything about her! So how can I get upset enough to swear vengeance… when I don't have any memories?"

The vengeance demons bring out a large crystal.

This is similar to the one that Anya offers Cordelia, in the Buffy show, but bigger. And much more powerful. It looks like it's the consolidation of every single vengeance crystal from every single vengeance demon there. Fused into this single gem.

It's the most powerful vengeance crystal ever created.

"Take it," the vengeance demons say.

Seo steps away, hands raised. "And make a wish? I don't think so. I don't even know this Dawn Summers-Peters. I'm not going to swear vengeance over her."

"You don't have to make a wish," say the vengeance demons. "Just take the gemstone. And you will remember everything."

Seo hesitates.

Then takes the crystal.

As Seo takes it, she cries out. All her memories of Dawn come flooding back, but not the memory of Dawn's death. All Seo knows is that Dawn should be here… but something's changed. Dawn isn't here anymore.

The only reason Donna could remember, was because her memory had already been so strongly tampered with… that it developed a resistance to this kind of memory wipe. Whoever had been covering their tracks hadn't counted on Donna's memory.

And Donna had confirmed… that time had been altered. So that Dawn had died, years ago.

The loss hits Seo, hard.

"Someone's tampered with history," Seo says. "They killed my aunt! In the past! They…!"

She struggles not to cry.

"Time has changed," the vengeance demons agree. "History rewritten. Dawn Summers-Peters… and her entire family… are gone, now. No one can get it back — except, of course, vengeance demons who can tamper with timelines."

Seo grips the crystal more tightly.

"It only takes a wish," the vengeance demons remind her. "And we can set this right."

Seo puts the crystal in her pocket. Then turns away from them. "No," she decides. Heads back to Oliver. "There's another way to fix this. A better way."

She opens the doors to her ship, and steps inside.

Only just having time to shout back at the vengeance demons, "And I'm going to find it!"

Then she dematerializes.

When she leaves, the vengeance demons all turn to each other. Asking the question that's on all their minds.

"Is it worth the risk?" the vengeance demons ask each other. "Will she do it?"

It's a huge risk.

The vengeance demons have combined all their powers into that one single crystal, so that it has the enormous power needed to undo Dawn's death, and reintroduce her memory back into the world. But if Seo decides not to make a wish, and to smash the crystal, instead — those vengeance demons would all turn human. And that'd wipe out most of the vengeance demons on Earth.

With no D'Hoffryn, there'll never be any more to take the place of those vengeance demons who have fallen.

"She won't smash the crystal," one of the vengeance demons assures the others. "You'll see. When she discovers the truth… she'll know that we're her only chance to get Dawn back."