After the third consecutive time Jim gets seriously injured on an away mission, Spock starts stress purring as soon as Jim mentions joining the away team. He stops after a few seconds, as soon as he realises he’s done it. It’s a mistake that leaves the entire bridge crew confused, but they brush it off pretty quickly. Spock, embarrassed, offers only a token resistance to Jim’s joining the away team, and vows never to let it happen again.
Three months later it happens again, and then again, and again. Spock finds it difficult to suppress the instinct - it’s vestigial, almost like goosebumps for humans, and he only catches himself doing it after he’s already started. So much for the famous Vulcan ability to control every single body function, Michael would tease.
It grows worse for a while after he thinks he’s killed Jim on the red sands of Vulcan. He starts purring every single time Jim enters his field of vision - not an entirely happy sound. It’s a combination of relief and guilt and the urge to stop Jim from ever leaving his field of vision ever again. Then, as always, he suppresses the urge, and grows quiet again. He’s never been so thankful as then that Humans don’t have as good a hearing as Vulcans; the crew has yet to suspect anything. Except perhaps Jim. But Jim has always been incredibly perceptive.
It never becomes as manageable as it was before, but then it rears its ugly head again when Jim is lost to the interphase in Tholian space and presumed dead. Again. Jim’s own mortality is distressing, and for some reason, it… doesn’t stop. He has a stress purr always in the back of his throat nowadays. He never wants Jim to leave. He never wants Jim to leave him.
He allows himself to stress purr out loud in the privacy of his quarters, when he’s off-duty, before he starts his daily meditation session. A part of him, that sounds suspiciously like his father, can’t help but consider it a failure of his Vulcan control. Another part, which sounds like his mother instead, says that if his body has an urge there must be a reason, and he’d better let it run its course if it will help him get better.
It does help him get better. It helps him so much that he includes it as part of his own routine - and that’s how Jim finds him one night, when he comes into his quarters for a surprise visit.
“Spock, I need your signature for - Spock, are you purring?”
Spock - who had been so into his routine he had given Jim permission to enter without even thinking - stops abruptly, the shame almost threatening to swallow him. He doesn’t answer.
Jim comes closer, awed. “I never knew Vulcans could purr.”
Spock finds his own voice again. It is pained. “It is not usually done in front of outworlders, sir.”
“Right, right. You’re all very private, and all that.”
Then Jim lights up, as if he’s just thought of something. “Can I ask you a question?”
“If you must,” Spock allows. Only because it’s Jim.
“Is the reasoning for your purring similar to that of Terran cats?”
“Very similar,” Spock concedes. Of course Jim is curious about it.
Jim, somehow, becomes at the same time bashful and delighted. Spock is almost blinded by that level of emotion.
“I know it is illogical, but Spock, are you purring because you’re happy? Well, satisfied? Here, on the Enterprise?”
Spock could not explain what happens to his body at that moment. He cannot lie, not to Jim. But he also knows that if he were to be completely honest, the truth would erase Jim’s wonderful expression, and as much as it is almost uncomfortable for him to look at such levels of naked emotions… it’s Jim’s. He cannot do that.
He decides to go for a version of the truth. “Happiness is an emotion and as you know, Vulcans do not feel. Nevertheless, I am indeed satisfied by my place on the Enterprise.”
It is not a lie, after all. He trusts that Jim will reach his own conclusions by filling in the gaps he’s left knowingly empty.
Jim smiles, a bright, shining thing that comes from the depth of his being and makes his eyes sparkle. For the first time, Spock wants to purr because of pure happiness, and it’s so unexpected and so pole-axing that he can’t keep it in. It slips out, a sweet, relaxed rumble that comes from deep in his chest.
It takes them both by surprise - Spock, because it’s so different from his normal purring, so deep and breathy instead of fast and desperate; Jim, because he clearly was not expecting for Spock to start purring now.
Spock finally manages to get it under control and the purring stops abruptly. The silence is deafening.
Then Jim comes close and gently - hesitantly, Spock wouldn’t dare say - puts his hands on Spock’s arms. Looking at him from under his eyelashes as he so often does.
“Are you… happy… here, with me?” Jim murmurs. Hesitant; yes. Spock is scared, and hopeful, and terrified, and yes, maybe… maybe he’s also happy.
Jim’s lips melt into Spock’s with a gentleness that makes Spock ache. Spock leans into him, but Jim retreats.
“Am I being presumptuous?” He asks. He’s checking in.
Spock shakes his head. “No, Jim.”
Jim smiles his sunshine smile, his heart in his eyes. “Good, then.”
And he kisses Spock again.
Spock is pretty sure that his purring can be heard from the Bridge, but he can’t help it. He’s happy.