Brain always slept much later than you. Mice were creatures of the night, and Brain was a case example.
You almost rolled out of bed, but decided to stay still when you heard scientists talking about your cagemate.
“It looks like he's blind in one eye, or injured, or something.”
“Did his tail always look like that?”
“He's hunched over, and he limps.”
“We've used him for tons of experiments; it's just now getting to him.”
“He's older than our other mice, isn't he? Maybe it's time for him to go.”
You gripped the blanket tightly.
“...I'll have an intern come in and euthanize him in the morning.”
You sat up and saw Brain's folder sitting next to your cage. One of the scientists wrote ‘TO BE EUTHANIZED’ on the front in permanent marker, and slipped it back in the slot next to your cage where it belonged. They left, and the lab's doors were locked. Half an hour later, Brain awoke.
“Good evening, Pinky,” he muttered through the drowsiness in his head, reaching for his cane.
“Oh… Hi, Brain.”
“I've prepared tonight's plan in advance…” He began to ramble, retrieving rolled blueprints. “... Are you listening? Pinky?” You were shook back into attention when he pulled on your ear.
“Narf! Oh, uh…” Your silence was slightly alarming.
“Brain, what does ‘euthanize’ mean?”
He let go of your ear. He looked surprised you knew how to say it correctly, much less hear it at all.
“To kill an animal humanely, usually for the purpose of ending its suffering. Why? Where did you hear it?”
A fib left your mouth before you could process the truth.
“On the TV.”
He forgot the discussion entirely, and went back to his scheme. You couldn't pay attention to a single word he said. Those scientists were going to kill him. A perpetual look of worry stained your face the entire night. He dragged you out of the lab for reasons you couldn't remember. All you did was watch him limp in front of you, gripping his cane like it was his livelihood. His confidence hadn't shrunk at all with age, he was the same mouse you had known your entire life. Your feeble, undeveloped mind could hardly process the thought of ‘What if his life is over?’ Were those two employees in lab coats making the right decision?
Time slipped away that night like sand through an open fist. You watched him modify a radio station's control panel, handing him tools at his request.
“Brain, how many more plans do you have… planned?”
“A multitude, Pinky. But if all goes well tonight, we won't be needing any of them.”
“How will we rule the world, with us being so old? Zort!”
“It doesn't matter how much time we have left. As soon as I am the leader of Earth, it will all be spent improving the world. I do all of this to make our planet a better place, Pinky, you know that.”
The plan failed when you handed Brain the wrong type of screw, and a copper wire fell loose during Brain's hypnotic broadcast. You received a bop on the head that was much weaker than you remember it being, and you both headed home.
“Head inside; I'm checking the mail.” Brain told you, and you climbed under the door. They couldn't kill Brain, you decided, he still had too many good plans! He was going to rule the world, or die trying. He wouldn't die by some some fancy science man in a coat telling him to.
But how would you stop them?
The files. You pulled out Brain's folder, which was right next to yours. Brain would be back any second. You moved quick. If you had been a smarter mouse, you would have thought of something better. But you weren't, and this was your best option. Opening both folders, you swapped both packets of paper, then closed them and shoved them back into the slot. When the intern comes in the next morning, he'll open the file that says ‘TO BE EUTHANIZED,’ and see your picture.
Brain came back with an envelope.
“It's just another credit card application for a Mr. Jamal Spelling.” He said, tossing it to the side. He briefly looked at your and his folders, which had obviously been messed with, but ignored it. You felt sick.
“We should go to bed. Goodnight, Pinky.”
You gulped, feeling a lump in your throat.
“Goodbye, Brain,” You muttered. He ignored the unusual response, assuming it was a slip of the tongue.
Morning came, and all was normal. You were both woken by the clatter of scientific equipment and human chatter. You got up and looked around, wondering where the intern was that would euthanize you. Not that you were complaining.
Hours passed, and still, no sign of the aforementioned intern. You ate breakfast in silence with Brain. He seemed lost in thought.
“Pinky, I… I have a confession to make.”
You looked up from the food pellet in your hands.
“Last night, I noticed that someone had tampered with our case files. And they had written on your folder that you were going to be euthanized, which is where you must have heard it.”
Your heart skipped a beat.
“I didn't… I couldn't let that happen. So I just threw away that folder and put all of your files into a clean one. Knowing how… Unprofessional and inefficient this laboratory is, the people working here will never notice.”
He looked at you, expecting a response.
“.... Haha! Narf!” Brain raised a brow at your reply, and was swiftly lifted in your arms.
“Why are you laughing?” He asked, concerned. You pressed him into a hug.
“I can go with you tomorrow night, and the next night, and the night after that!”
He was dumbfounded; his prediction was that you'd be depressed at your brush with death.
“Huh… Well, I am glad to have you come with me tonight.”
You bounced with anticipation as you asked your favorite question.
“Poit! Why, Brain, what are we doing tonight?”
“The same thing we do every night, Pinky; try to take over the world.