#christopher ibrahim

To start, I take a very strict "canon is whatever the hell I want it to be" approach which informs most of my thoughts, but before throwing it out the window, a few canon notes and timelines:

First let's establish the age gap: there are conflicting birth years for Sarah and none for Kate that I can find online, but averaging those with Lis Sladen's actual age and using Jemma's actual age as Kate's, we can assume that there's an age gap of 15-20 years (ymmv, I like to think closer to 20, especially since Sarah's birth year doesn't necessarily reflect her age given her time bouncing around in the TARDIS).

They only ever shared screen in an unofficial movie made in 1995, roughly twenty years after Sarah left the Doctor, and they would've been in their mid-late 20s and 40s (roughly) respectively. Their interactions were friendly, if not downright flirty, and it's ambiguous as to whether or not they'd met before; Sarah introduces herself, but imo it gives the vibe of "we've met before but you might not remember", and I think either interpretation works.

This movie is when Kate finds out what her dad's actual job was and is her introduction to the concept of UNIT (and the rest of it).

Sarah's in a pretty good place when they meet and the first couple of years after, but starting around 1997, the following decade is a shitty one for her. She loses a lot of loved ones (including her aunt Lavinia, who was her only surviving family, and both Harry Sullivan and K-9, her last links to her time with the Doctor); she's targeted by a genocidal cult; and she's betrayed quite horribly by one of the few people she's let herself become close to. By the time she reunites with the Doctor in School Reunion in 2007, she's shrunk her life into something quite isolated and depressing. At this point she's also deeply distrustful of UNIT and is much more wary of the militaristic tactics that she was unbothered by (and participated in) during her years working with them.

We know very little about Kate's life during that time except that she joined UNIT around 2004 with a goal of trying to re-prioritize science and discovery, though as years go by she and UNIT both become far more militant than it ever had been before.

Technically, Sarah dies at some point but I reject the canon timeline; Lis Sladen would be about 80 years old now, and as far as I'm concerned, Sarah lives to be 110 and is still drinking spritzer's in Donna's backyard with the Fourteenth Doctor. 

I like to imagine that Kate met Sarah a few times when she was young, and maybe had a little crush on her (and maybe a tiny bit of jealousy because she clearly knew her dad well, while he was never around for Kate?). But they were fleeting meetings, and they didn't properly connect until Kate was pulled into the world of all things DW and came to learn the truth about her father. Which would have been bumpy, right? We know they repaired their relationship, but I think Kate would have struggled a lot with the scale and scope of the secrets her dad kept all her life and it would have taken time for that trust to build. Sarah, though, has so few people she can share this part of her life with, and she was tied so deeply into Kate learning the truth, so I think they'd have clicked quite quickly and that Sarah would have encouraged both the relationship between Kate and the Brig and also would have been an invaluable source of knowledge and connection for Kate in that time.

So we have a honeymoon period, right, where they become close and Kate's a relatively young woman who's diving headfirst into this world that's brand new to her and she has this smart, lovely, older woman (who she's coming to be slightly in awe of, the more she learns about who Sarah is and what she's experienced) as her friend guiding her through this. And Sarah's happy to, she is. She thinks Kate is brilliant, all the best parts of her father. She sees a lot of herself in Kate, too. Herself, back when she was younger, and life hadn't beaten her down.

And she knows Kate has a bit of a crush on her, and it's flattering, really. She's approaching 50, and this beautiful young woman is interested in her, and is one of the only people in the world she can be honest with. But it's just friendship. Kate knows that, and Sarah knows that, and for a couple of years, that's fine, and they have a close, easy friendship. If there's a little bit of tension sometimes, it's easy to move right past.

Then K-9 dies. Then Sarah's Aunt Lavinia dies. And it's a terrible idea, it can't possibly go anywhere, but one night they're having a drink and there's that tension again, except this time Sarah is tired and lonely and when Kate moves closer, she doesn't push her away.

It still isn't anything. Sarah's very clear about that, not even considering the obvious factors working against them as two women in the late 90s/early 2000s. She isn't interested in a relationship, and even if she was, it couldn't be with Kate. But it can be something else. And it is, for awhile. It's even good for awhile.

But Kate's young, unjaded, and has an endless horizon in front of her. UNIT has become unrecognizable to Sarah, and Kate wants to go in and change it. She wants Sarah to join her. She's so optimistic—driven—and Sarah's tired.

Kate moves towards UNIT, and her world opens up even as Sarah's closes in. Kate's writing the future; Sarah's haunted by her past (literally, as the Crimson Chapter targets her). The long conversations they used to have about how to make the world better, about the wonders of the universe and their place in it, become arguments. It's not that Sarah doesn't believe in Kate—she does, she says, of course she does, but...

But Kate's in love. Of course she's in love, how could she not be? And Sarah's furious at herself for letting this happen, because she's known all along that whatever she feels for Kate will only hurt them both. And it does. Easy friendship turns into a tense affair, and as the years pass, it turns into nothing much at all.

It doesn't end, exactly. It fizzles. Kate meets someone and works her way up through UNIT. Sarah reunites with the Doctor, adopts Luke, and starts over. They cross paths once in awhile and it's fine, totally fine, except that Kate's hiding a heartbreak that Sarah recognizes, and Sarah's filled with shame. She pulls even further back. She gives Martha and Mickey Kate's contact information, she calls UNIT in to clear up as needed, but largely, she steers clear. 

Years pass. The Brig dies. Kate isn't surprised to see Sarah at the funeral, but she is surprised by the Sarah she sees—older but softer, surrounded by family, quietly telling Kate that she's sorry in a way that leaves no doubt she means more for more than just this most recent loss. And Kate is older now, too, becoming harder, spending her days with a young woman that stares at her with stars in her eyes that she doesn't quite know what to do with. Sarah writes her a note afterwards, short and sweet—it doesn't say much but it says enough. 

It's a little easier after that. Kate meets the Doctor, sees how wonderful he is, and also how much damage he could do to a person who loved him the way Sarah had, the way she'd loved Sarah. The way she's starting to fear she might be loved by someone she can't give very much to, someone she's bound to hurt. 

Luke works with UNIT for awhile, and Sarah opens up to collaborating with them more for a time, but UNIT isn't in the shadows anymore, and Sarah doesn't want anything to do with the spotlight—especially as she sees what it's shining on. Kate grows harder and harder, and there are so few people she can let see any other side of her. 

But there is Sarah. 

They work together a couple of times—purely professional, but it feels easy like it did twenty years earlier. Eventually there comes a night where Kate, exhausted and raw the way Sarah had been that night that changed everything years ago, gives in. She calls Sarah.

It's a cup of tea and nothing more, but it's nice. There are so few people Kate can talk to now, the weight of the world and duty heavy on her shoulders, but slowly Sarah becomes one of them. It's not often, but on rare occasions, they talk.

They don't talk about the past. Not explicitly. It breaks Sarah's heart a little, though, to see what the world has made of the young woman she met all those years ago. But for all that Kate is older and jaded, she's still young, at least to Sarah. And Sarah knows a thing or two about walling yourself off, and the joy and pain of the right people at the right time dismantling those walls brick by brick. Kate's older than Sarah was when they first met, but not by much, and Sarah had no idea how much of her life hadn't even started.

"The things you've done are pretty impressive," the Doctor told her once on one of the worst days of her life, nearly fifteen years before, which was fifteen years after she first met Kate, "but oh, the things you're going to do..."

And she knows how he must have felt, how proud and sorry he must have been.

As for Kate—

There was Osgood, for a time. Maybe they were something, maybe not, but Kate distanced herself regardless, understanding Sarah more than she ever expected to in those moments of pulling away. Then there was Christopher. She was tired by then, the loneliness heavier than ever, so she gave in; she let herself have him, even let him have a little of her. She knows she breaks their hearts and hates herself for it, but she also knows she wouldn't have traded her time with Sarah to be something less broken.

Time moves forward. And backwards, and sideways, and backwards again, even disappears all together for awhile, but mostly it moves forward.

Maybe Christopher dies. Maybe Kate mourns him, and regrets getting involved with him nearly as much as she regrets not letting it be more than she let it. Maybe she vows never again and closes herself off even tighter.

Maybe he doesn't. Maybe he survives, and those walls start to come down. Maybe they can be something in a way she and Sarah never could.

Maybe he dies, and Kate mourns, but runs with her regrets and all the things she wishes had been different only to realize that maybe they still can be—not with him, but with someone else. Maybe she takes a step back and reconnects with her children, with the world outside of UNIT, the world she's dedicated herself to but hasn't truly been a part of in decades. 

Maybe she meets someone, or reconnects with someone she left behind a long time ago. Not Sarah—not in that way, at least, though Kate will always be just a little bit in love with her—but. Osgood, perhaps. 

Regardless, there comes a time when she's no longer alone in the way that she was for a long time. And when that time comes, she takes their hand and says, "there's someone I'd like you to meet."