They explore the universe together.
They can go anywhere, now. Everywhere. They can go places even a TARDIS could never take her, places she could never have survived.
They stand, the two of them, in the heart of a star, and watch it burn around them. The fusing nuclei are too small to see, to strange to understand.
But Bill sees them. She understands them. And it's beautiful. It's better than any physics lecture. It's real.
"Where should we go next?" Heather asks, and smiles.
It was always otherworldly, her smile, even before she changed. Bill loved it then. She loves it now. She can't imagine how she ever felt afraid of that smile, even for a moment. It was Heather. Always Heather. Whatever Heather might be.
"Have you been to see dinosaurs?" says Bill.
Heather shakes her head.
"I want to see dinosaurs!"
Heather smiles again, and takes her hand, and there they are. As easy as asking.
It's nothing like Jurassic Park. It's way, way better. It's hot breath and life and color and movement in every dimension.
A lot of them have feathers. You hear that, on science programs on the telly, but it isn't the same as seeing it. As knowing it. As reaching out to touch the soft prickle of an actual bloody dinosaur feather, to feel the shape and the pattern of it echoing somewhere inside the substance you've become. It's a different kind of knowing.
There are creatures here that Bill's never seen, never heard of. Animals that never made it to the Natural History Museum. Lost to time and the limits of human knowledge.
But they're here. They've always been here. Waiting for her to touch them.
She sees her mother, once. Didn't even plan to, really. But she could feel something like a tug inside her, like the stirring of a memory, drawing her somewhere, drawing her towards.
They can go everywhere. Even to the places she didn't know she needed to go.
They find her mum, young and laughing, holding her hand up to block her face from a camera. Wanting to live here, in the moment, not captured still and lifeless for eternity. Bill never understood that before, why her mum wouldn't want her picture taken. She understands it now, the way she understands feathers, the way she understands fusion.
They don't speak to her. Don't interfere. It wouldn't seem right, somehow. But they watch, and they understand.
Like the dinosaurs, she was never really lost. She was here all the time. Laughing.
They see the Doctor once, too. She looks different – and, wow, if he'd looked like that when Bill knew him, things could have got complicated – but Bill knows it's the Doctor. Knows by the fizzy alien energy inside her and the extra heart. By the way she's talking to the people with her, the way she looks at them. By the door she's stepping through, of the police box that isn't a police box. (It is a knock through, Bill sees now. A knock through right into the walls of the universe. She can see how it works now, see what it is, and she can't help but smile at how right she was, the very first time.)
"Do you think she's from before?" says Bill, "Or after?"
Those concepts don't mean what they once did, but Heather understands her. Heather always understands.
"I'm not sure," she says. "We could ask?"
They watch the TARDIS door close, watch three humans and one Time Lord walking down the street, somewhere deep into what was once Bill's future. "I think after," Bill says, remembering the Doctor's body, lying still. Remembering loss and hope and the sense that, surely, nothing was ending. "I want to believe it's after."
They don't ask. They let the Doctor go on her way, and they continue on theirs.
They play hide and seek. The longest, biggest, most amazing game in the universe. Bill finds Heather among the wisps of a nebula, in the tail of a comet, in the bioluminescent depths of an alien ocean. Heather finds Bill among the stalls of an alien marketplace, at the top of a mile-high tower, on the plains of an African veldt watching her ancestors learning how to speak.
When they reunite, they embrace, melting into each other like warming ice.
They love each other, in their own, new way. They make love in woman-shape and water-shape, and blur the edges of their selves together. It's wonderful. It's amazing. "Proper sci-fi sex!" Bill calls it after the first time, and they tilt their heads back and laugh in harmony.
Time doesn't pass. The Doctor taught her that, once, in a lecture hall full of eager human ears, eager human minds. Time doesn't pass. Everything still exists, always, where it has been.
But even so, we change.
"I think it's time," Bill says.
They are standing atop a cliff, watching a sun set. Its brightness is crisp and clean, undimmed by atmosphere of any kind. It's bright enough to blind, if blindness were a thing Bill was still capable of.
"I see," says Heather, and the star in her eye glints back at the one in the sky, as if they are the same.
"I've loved this," Bill says. She doesn't need to breathe, doesn't need to swallow, isn't even speaking with sound waves at all. But still her voice comes out ragged, and with difficulty. "I love you. So much."
"I know," says Heather, and takes her hand. Which isn't a hand, not really. It hasn't been a hand for a long, long time.
"But I'm not like you," Bill says. "I can't do this forever. You always just wanted to leave. I wanted..." She squeezes Heather's hand. It's not a hand. But it still feels like one.
"You wanted to leave, and then come back," Heather says.
"Yeah." Bill swallows. Or does something with the memory of swallowing. "Yeah. I've loved being this. But I never wanted to be.... I dunno. Not human."
Time doesn't pass. But whatever it is they experience instead of time, it's changed her. Is changing her. She can feel it. She is becoming... something. She can feel the universe dissolving inside the substance of her, until she can see the day coming when she will be more it than she is herself.
She doesn't want to be a liquid any more, taking on the shape of whatever surrounds her. She wants to be solid. She misses heartbeats. Misses sunrises and sunsets happening one after the other, all in order. Misses being simply, only Bill.
She is only here in the first place, really, because she didn't want to die as something other than a human being.
"Bill," Heather says. "I understand. It's okay."
"Is it really?" There is a tear on her cheek now. Heather smiles and captures it with her finger.
"Of course," she says. "I knew this time would come. I know you."
"You do," says Bill. No one has ever known her as well.
Heather smiles. "And nothing ever ends. You know that."
"Yeah," says Bill. "Sometimes it feels like it does, though. Will... Will I ever see you again?" Because the future does exist. If it doesn't, how can we have hope? And the hope Bill feels is real. It trembles in her voice.
"If you like," says Heather. "I can come back, when you want me."
"All I have to do is cry?" She's doing it now. Again. But she's laughing, too.
"All you have to do is ask," says Heather.
Bill nods. "Thanks," she says. "Thank you. I... I don't know what else to say, I..."
"Don't say anything," says Heather. "You don't need to. Just be. Just be Bill."
She reaches out, and touches Bill, and...
She is standing on grass, looking up at the sky. Above her, the sun is too bright to see. Around her, she can hear birds singing. She can't see inside them. The molecules of their feathers are, once more, a beautiful mystery to her.
She draws air into her lungs. It feels familiar. It feels like breathing.
She closes her eyes. Sun in her face, air in her lungs, birdsong in her ears. All still here. None of it is going anywhere.
She opens her eyes again and looks around. She's back at the university.
She can feel the future ahead of her. She can't see it, can't touch it, can't travel through it. Not anymore. But she can feel it, anyway, the wide-open possibility of it. Maybe she'll teach. Maybe she'll tell her story. Maybe she'll travel, see what the world looks like through proper human eyes. Maybe she'll sit beside Heather, some days, and enjoy feeling the differences between them when they touch.
Maybe she'll find someone to grow old with. She hopes she will.
But for now...
She turns, and finds the canteen behind her, right where she left it.
For now, she thinks she'll go and have some chips.