The Great Mouse Detective or Fawlty Towers - Take your pick.
Note: No rodents were harmed in the writing of this drabble.
Sherlock sets the cardboard shoebox down on the counter. It's shaking slightly, and there are worrisome scratching noises emanating from it. John raises and eyebrow, staring pointedly at it.
"Sherlock, what the hell is in there?"
"A mouse."
"A mouse!"
"Mrs. Hudson found it and asked me to trap it for her." Sherlock trails off. John waits for him to finish explaining why the mouse is now in their kitchen, in a box. When no answer is forthcoming, he pushes the issue.
"Okay, that was nice of you. But why the hell did you bring it upstairs?"
"Oh, I thought I could experiment on it."
John splutters. "No. No you can't. Sherlock, it's a live animal!"
"Oh, don't worry. It won't be when I'm done."
John takes a deep breath, trying to calm himself. "Absolutely not. You're not killing an innocent mouse in the name of science."
"Oh, I'm sure Mrs. Hudson would confirm that it's not innocent. Why does this bother you so much?"
"Because it's just a poor mouse!" Gingerly, John opens the box and cups the tiny, quivering animal in his hands.
"Maybe we could keep him around for a bit. Get a cage. You know, like a pet."
Sherlock snorts. "A pet, John?"
Smiling, John pats the mouse on the head. "Yeah. We can call him Basil."