Please imagine Spock finding a stowaway on board the Enterprise.
It’s three weeks after their recent refit on Earth and they are currently neck-deep in operations near Romulan space, and there’s a child in the cargo bay.
Spock finds her wedged between two crates amidst a sea of padds and crumpled up food ration packaging. She was asleep when he got there but before he can so much as grumble disapprovingly she leaps up and looks about ready to fight. He decides to attempt to diffuse the situation before calling security. After all, she is only a child.
She immediately says she’s not a child. She’s fifteen. Way above childhood, for humans, and he starts to grow curious. Because what is a fifteen-year-old human girl doing on the Enterprise? Has she been living here for those three weeks? How has she remained hidden? What is her name?
She says her name is Joanna. Joanna McCoy.
And that’s…fascinating. And wholly unexpected. Now that she says it, he can see the family resemblance in the grimace of her mouth and the glint of her eyes. This is McCoy’s child; a child Spock has never heard of before. He asks why she is here and she tells him that she wanted to be closer to her father. That her mother no longer has any use for her. That she ran away.
(And Spock understands something about running away from home.)
When she begs him not to tell anyone, he obeys without fully understanding her logic.
She isn’t ready to see McCoy yet. She’s been watching him on sensors and sneaking around the jefferies tube near the sickbay. But now Spock takes her back to his quarters and gets her some real food and a place to sleep. He meditates before his altar, thinking. Considers his options.
He decides that he will not turn her over to the captain. The Captain is far too busy dealing with the Romulans, anyway, and does not need this distraction. Spock will inform him when it becomes relevant. In the meantime, perhaps he can learn something of how to interact with McCoy by interacting with his daughter. So they begin to spend more and more time together as he makes sure she is keeping up with her studies (despite her protests) and she makes sure he is eating and sleeping (despite his protests). She begs him to tell her stories about what her father does during the day, and Spock agrees begrudgingly.
Unfortunately, most of the stories are about the two of them fighting.
For reasons he cannot fathom, Joanna seems to find this very amusing.
She begins to give him advice on how to handle McCoy, and to Spock’s surprise it works! He can now defuse arguments with McCoy 87% of the time, and the other 13% are enjoyable. Almost loving. And he even catches McCoy smiling at him occasionally. Spock finds they are growing closer. That he enjoys his time with McCoy; this man who is just as bright and intelligent and fiercely loyal as the daughter he has never known.
Spock is beginning to realize that he has feelings for McCoy. Romantic ones. Domestic ones. He begins to argue with Joanna that they must come clean about her presence, and if he is honest with himself he will admit that it is because he wishes to share her successes with McCoy. She is quite adept at interstellar physics.
But she is afraid that McCoy will not love her (and Spock understands something of that fear, too).
It isn’t until the Romulans get tired of following them around and actually attack that things get a little dicey. Joanna is injured and Spock rushes her to sickbay, where McCoy remains calm and cool and collected right up to the point where she is out of danger, when he promptly faints. When he gets up, he spends three hours yelling at Spock before he starts crying.
Spock holds him through it, and McCoy says she’s in the best health he’s ever seen. He thanks Spock for taking care of her. McCoy is so happy that she is here, yet so fearful for her. Fearful that he will be a bad father to her.
Spock says, he is here. He will always be here beside McCoy.
When Joanna wakes up, the three of them figure out where to go next. Together.