When Mycroft entered Eurus's cell, he didn't begin speaking right away. Instead he checked all the cameras and sensors, making sure not only that they would not record the conversation, but that, if examined, they would play back appropriate counterfeit footage with every sign of authenticity.
"Good god, brother, what are you doing?" Eurus exclaimed. She was genuinely surprised. This would have been perfectly normal behavior coming from literally anyone else she communicated with, but Mycroft? Mycroft never broke the rules. It was one of his numerous intensely irritating traits.
Mycroft ignored her until he'd finished double-checking every recording device in every corner. He was very thorough, another irritation. But finally he finished, turned to her, and said, "Eurus, I need your help. It's about Sherlock."
"You are coming to me because of Sherlock?" This was, quite simply, the most surprising thing that had ever happened to her. She was never surprised. Especially not by her irritating older brother.
"He just shot Charles Augustus Magnussen in the head in front of an entire team of police. It was quite justified, but there is no chance the government will permit him to go unpunished. They are entirely set on sending him to his death in a foreign nation." Mycroft spoke in a flat, matter-of-fact tone, his face and body giving absolutely nothing away.
Of course, Eurus could see right through him. Having gotten over her surprise, she regarded her brother with excited amusement. What he didn't realize was that she had been brainstorming ways to get Sherlock's attention, and that he had just handed her on a silver platter an opportunity far grander and more hilarious than all the alternatives she'd considered. And he was in an emotional state perfect for a little sisterly torment. It really was Christmas.
She would start simple. "And why," she asked, voice high and mocking, "should I care?"
"I know you care," Mycroft replied evenly. "I know you still think you'll get a chance to play your own game with Sherlock one day, and you'd hate to see him taken out before it's your turn."
Eurus giggled. It was so cute, how Mycroft honestly had no idea how close she was to having her challenge to Sherlock ready. All she needed was an extravagant gesture to begin with. "Oh, yes, don't you worry, Sherlock and I will have so much fun," she said, singsong. "You, though, you know I don't need you. You know we don't need you, Sherlock and I. You were always the one that nobody needed. Too old for us, too grumpy for Mummy and Daddy, too chubby for everyone."
Mycroft continued to keep every trace of micro-expression off his face, which was proof that the knife had gone in.
"The point is," Eurus continued, "no amount of video-tampering will save you if I tell your employers about your treachery."
Mycroft held her gaze, steady as a great tree in the gathering wind. "I think you believe that I am more useful to you in the center of the British government than disgraced and in jail. And if I'm wrong...Sherlock will still be safe."
"Oh, are we playing the selfless big brother again? How old. How repetitive. How tiresome. Do you ever wonder what it is you're trying to prove to yourself when you act like this? When you obsess over Sherlock's every little scrape and boo-boo, when I ask for a violin and you give me a Stradivarius, what are you hoping to buy?"
Mycroft wouldn't respond. "I can tell you've decided to help. Get on with it."
Eurus rolled her eyes. "Really, the answer should be obvious even to you, if only your mind weren't so tied up in the law-abiding straitjacket."
"Give me a hint."
"Fine. Your bosses want to send Sherlock away because they think they no longer need him. You have to show them just how much they still need him."
Mycroft stared. She could almost hear the creaks of the gears turning in his head, painfully processing the kind of solution he never, ever considered. "Bring back an enemy that-that only he can deal with," he said slowly. "Moriarty."
"You have interrogation videos," Eurus said. "You have access to the whole nation's broadcasting infrastructure."
"If I could put Moriarty on every TV screen-" Mycroft murmured. "I don't have that much access. Not enough to slip it in undetected."
Eurus beamed. "I can help with that," she said, and told him how.
It was done. Brother and sister gazed at each other across the reinforced glass, which Mycroft did not know could now be removed as easily as a sliding door, and wondered at the strange and unexpected things that lurked behind each other's eyes.
"You're more alive than I thought," Eurus said, smirking. "Perhaps we'll include you in our game after all."
Only now did Mycroft dare allow expression-specifically, a scowl-back on his face. "There will be no game," he said, "except over my dead body." He turned to leave.
"I wonder," she called out after him, mostly to frustrate him but also slightly because it was true, "whether things could have been different if I'd known this when I was five."
"Known what?" he asked, not turning around.
"That just occasionally, at the oddest times, you can find it in yourself to be a very good brother."
Mycroft froze briefly at her words, seemed to shake them off, and walked away.