The first time John fell down the stairs, Sherlock disregarded it. They'd been chasing traces of Moriarty all the way across central London since early morning, and despite his normal superhuman tendencies, Sherlock had to admit he was tired. He could only imagine how the army doctor was feeling, what with his normal desires for sleep and food. As he heard the thump that he could easily deduce as a middle-aged male of average height and weight falling down the stairs, he called out a terse "John?" and received a grunt in reply. It was enough. He turned back to his laptop. They did not speak of the incident, though Sherlock noticed that John seemed to clutch at the banister a little tighter than before when he went up the stairs, and several nights that week he merely dozed on the couch.
The second time, a week later, could not be ignored. Sherlock was sitting in his chair, plucking absently at the violin, when a series of staccato thumps rang throughout the living room. Sherlock correctly recognized that the sound of the thumps indicated several sharp raps to the skull, and he leapt up, dashing to the foot of the stairs to assess the damage to his flatmate.
"John?" he asked cautiously, catching the doctor's wrist for a pulse. "John, can you hear me? I need you to speak up if you can hear me."
What is wrong with him? And why didn't I notice? Sherlock wondered as he squeezed the limp hand of his assistant, overwhelmed by the weight of his relief when the hand squeezed weakly back.
"Sherlock?" John asked, his voice faint, as if he were thousands of miles away. "Did I fall again?"
"That looks to be the case," Sherlock replied, his voice unusually gentle. "Should I call for an ambulance or will a sack of peas do? I'm sure we have some sort of frozen vegetable. If not I can pop one of the arms I'm working on in the freeze-"
"Sherlock." John's voice had regained its comfortingly steady tone. "We need to talk."
"Well, obviously. I was in the middle of a particularly intense thought when you fell down an entire flight of stairs and broke my concentration," the detective snapped, holding his hand out for John to grasp. He paused. "What has gotten into you? Sudden gait ataxia and" he leaned close to sniff John's breath, "several bouts of vomiting poorly masked by breath mints suggesting you've been suffering from severe nausea – I'm assuming these were after the headaches you've complained about – as well as," and here he raised John's left eyelid to look carefully at his eye, which gave an involuntary jerk, "apparent nystagmus and papilladema –"
Sherlock's eyes suddenly widened, his mouth dropping open, as John regarded him quietly. "No – it can't – John, no, that's impossible, not – "
"Yes, me, Sherlock," John replied, struggling to form crisp consonants.
"But the life expectancy is less than twelve months –"
"And I've known for about a month now."
"Why didn't you tell me?" Sherlock snarled, pulling the army doctor up from the floor in one swift movement, nearly pulling them both off-balance.
John chuckled; Sherlock suddenly realized that his voice sounded slurred, yet another symptom he had failed to notice because of his rapt attention to his work. "It's kind of a hard thing to bring up. There's no set guidelines for telling your colleague –"
"Your friend," Sherlock corrected, his eyes averted.
"- that you have a brain stem glioma. Should I have just slipped that in when I asked you what you wanted for dinner some day? 'Sherlock, Chinese or Angelo's, and oh by the way I'm going to be dead in less than a year'?"
"Well, I would have liked some warning," Sherlock replied stiffly. John noticed tiredly that the other man had not moved away, or even released his grip on John's jumper.
"And now you have it. Can we discuss this later? It's late."
"John, I – "
"I get it. I was planning on admitting myself to an assisted-living facility as soon as I could figure out a way to afford it. I was just waiting for the symptoms to get bad enough to warrant it." The doctor's voice sounded colder, colder than Sherlock had ever heard it, even after hearing him rage against criminals and hiss at murderers. The tone, plainly, terrified Sherlock.
"Don't talk like that. You're not going anywhere. I'm going to take care of you."
"What? No. Sherlock, you can't even keep a houseplant, much less someone who is very close to losing control of all his bodily functions." John was tiring; his voice was more slurred than before, his brow furrowed in the effort of concentration. Sherlock noted with a stab of guilt that he'd never before been able to notice when John was tired. Apparently, it took the realization that their time together was much, much shorter than either intended to recognize just how much of a mystery John remained. And as much as Sherlock loved a mystery, this was not the kind he wanted to solve.
"John." He hoped his voice was as firm and commanding as he didn't feel at the moment. "You are going to remain here. I am going to stop taking cases unless they are of exceptional interest. And I will take care of you. Consider it a … gift for your service. I will ensure that you are in the least amount of pain possible for the shortest amount of time, and that your final months are as pleasant as I can manage. Understood?" He squeezed the doctor's hand, hoping to impart some sense of kindness and empathy by the touch, knowing that his sterile, clinical voice could not convey the firestorm raging in his heart.
The gesture, apparently, didn't go through. "So this is like the gold watch they give a banker when they retire?" John spat. "Shoving me off this mortal coil with a pat on the back? I don't need your pity, Sherlock."
"This isn't pity," Holmes growled. "This is a socially ignorant but intellectually brilliant man doing his best to show his friend that he cares, and that he will remain by his side for as long as he is needed, regardless of what that may do to his sanity, pocketbook, or reputation."
John looked nonplussed, his eyelids sagging slightly as he looked up at the taller man. "So… you do care?"
Sherlock couldn't help himself; he laughed, a short bark more than a true chortle. "Your deduction skills are not at their finest tonight, John."
The army doctor looked away. "They're at their finest they will ever be. This is the downhill, Sherlock."
The consulting detective stilled, his face impassive but his eyes filmed over with a fine veneer of liquid. "Right then. Do you need help up the stairs?"
"That's very kind of you."
"Actually, disregard that. You can sleep in my bed. I'll take yours."
"… are you sure?"
"Don't be ridiculous."
"No weird experiments lying around in the covers?"
"Of course not." He paused. "Actually, let me check." Ducking into his room, Sherlock returned moments later carrying a leg. "All yours."
"Thanks," John replied quietly. "Really."
"It's no matter," Sherlock responded, biting his lip. He watched John pad slowly into his room, closing the door carefully behind him, and then he ran up to what was formerly John's abode, not trusting himself to hold it together for much longer.
His assumption proved correct, and he'd barely passed the threshold of the room before the dam burst open and tears poured down his face. Sherlock landed squarely in the middle of the bed, holding his legs to his chest and rocking back and forth, sobbing into his knees. He couldn't remember a time in his life where he had been so emotionally invested in the health and safety of another person – in fact, he realized, he probably would never be in such a position again. Surely there was not another John in the universe, a constant calming force who could defend him from both enemies and himself, a steadfast companion with the nerves of a soldier and the heart of a doctor. And if such a person existed, Sherlock reasoned, he'd never want to meet him, because then he would be left with nothing, instead of only half a heart.
There was only one thing to do in this situation, he decided: admit his weakness to the only human being other than John who would possibly care. With shaking fingers, he pressed the number into his phone and clutched the device to his ear, still weeping.
When the call was picked up, Sherlock lost it all over again.
"Mycroft," the great detective sobbed. "John."