A/N: Warning for major character death.
"Better now?" John asked.
"Still got that dreadful taste in my mouth," Mycroft admitted with a grimace. He raised the carton of apple juice and sipped at the straw, creating a hollow sucking sound.
"That sounds empty. Here." John grabbed another chilled box from the insulated bag on the bedside table and handed it to him. Mycroft grabbed it eagerly, poked the straw through the foil, and took several deep gulps.
"That bad, huh?"
"Dear God." The older man wiped his mouth and shuddered. "Worse than that castor oil Mummy used to make Sherlock and I take."
"I've heard it's not too pleasant. That's why I brought the juice. To wash away the taste."
"Thoughtful as ever, John." Mycroft took one last swig and handed the carton to him. Then he adjusted the lapels of his dressing gown and leaned back against the mountain of pillows. "Thank you."
John tossed the deflated box into the small bin he'd brought to Mycroft's town house; its other contents included the plastic spoon and cup used to mix and administer the medicine, as well as the nitrile gloves he'd worn while doing so. Then he resumed his gentle grip on Mycroft's wrist, measuring the pulse. It was still steady, but he thought he could detect intermittent flutters beginning.
The house was silent except for their voices and the rain beating against the windows on all three stories. The servants had been sent out on deliberately fruitless errands. Even the surveillance cameras did not click or whirr, having been shut down an hour before.
The elder Holmes took several deep, exploratory breaths. "For something that tastes so horrible, though, it doesn't act very quickly, does it?" He sounded almost reproachful.
"A few minutes at the most." John forced himself to sound casual. "What's the rush?"
"I'm a Holmes, John. There's your answer."
"Christ. You sound like Sherlock."
Mycroft smiled and drew the covers up over his waist. "Did I ever tell you that Sherlock once became obsessed with the idea that he was adopted? He was thirteen, I believe, and wrote a computer virus that unlocked every sealed adoption record in England and sent it to him via e-mail. He was so sure that we couldn't actually be brothers, because we were so dissimilar."
"Really? Even with your matching intellects?"
Mycroft shrugged. "He saw nothing exceptional in our abilities. Still doesn't. As far as he's concerned, he and I are normal and the rest of mankind are blithering idiots."
"Yeah, he loves getting that point across."
"We did turn out to be compatible in a way I'd never have predicted, though."
John responded only when he could trust his voice. "That's true."
That compatibility came to light only after Sherlock's heart disease had progressed to crisis level. Dozens of potential donors had been covertly tested at the morgue with Molly's assistance and found to be incompatible with Sherlock's unique physiology. Then one rainy evening (almost as rainy as this one) last week, Mycroft had John picked up and brought to the Diogenes Club, where he presented him with a donor profile. When John read it, joy and hope soared through his long-depressed system and he exclaimed, "This is… well, it's a miracle! We'll need to retrieve the heart and schedule Sherlock's surgery immediately. How long has this person been dead?"
"They're not- that's the problem," Mycroft replied mildly. "That's why I need your help."
"I don't follow."
Thus began a tortuous conversation, one that sent John back to Baker Street hours later with tears streaming down his cheeks and a bitter promise on his conscience. Now he was here to fulfill it.
The elder Holmes yawned. Beneath John's fingers, his pulse slowed. "John, you're positive that this prescription won't damage my heart in any way?"
"No. It won't."
"Good." Although Mycroft remained alert, John's experienced eyes detected a gradual loosening in his limbs and facial muscles. "How was Sherlock when you saw him at Bart's today?"
"Not in the greatest condition, but after the transplant, he has an excellent chance at recovery. It will be a long recuperation process, though."
"Which you shall help him through, I trust."
Mycroft smiled. "Thank you, John." Then his expression hardened slightly. "Remember our bargain."
"I will." He knew what he was supposed to say when he called 999 after it was over. Came to update him on his brother and found him in bed. Overdose. Note on dresser citing work-related pressures. Suicide. Molly, who'd tearfully pledged her assistance, would extract his heart and preserve it for immediate transplant, with Sherlock never knowing who the donor was. (As soon as the operation was complete and he was stabilized, he'd be told that Mycroft was dead, but not the real reason why.) "You're a hero, you know."
John was talking to convince himself as much as the man dying before him. He had not wanted to do this. He hadn't wanted a choice between Mycroft's or Sherlock's life to even exist. But it did, thanks to an older brother's persistent love.
"I want it to be you, John," Mycroft had said while the Diogenes fireplace threw warming light over John's shock-chilled form. "But if you feel you can't, I'll make alternate arrangements and just rely in your discretion. For Sherlock's sake if not mine."
Faced with that indomitable will, John had agreed. If Mycroft was going to die, he deserved better than a stranger during his last moments.
"All part of being a big brother," Mycroft answered.
"No, it's not. You're giving Sherlock another chance at life."
"For the millionth time since he was old enough to get into trouble." There was a definite slur in his speech now, and his lids were flickering rapidly. "Hang on, John. I can feel it working…."
"It's okay." John couldn't stop the thickening in his voice, but he refused to cry until later. He raised his fingers from the older man's weakening pulse to his hand, and clasped it tightly. "I'm here."
Mycroft's eyes finally closed. "Good... night... John..."
"Good night, Mycroft. Sleep well."
John slid up the bed and gathered Mycroft in his arms, resting the man's head on his shoulder and finally letting huge, wracking sobs break through. Sherlock would never be allowed to know, and John would never be able to forget.
Finally Mycroft's shallow breathing stopped, allowing him to save his younger brother's life one last time.