Prompt from V Tsuion: A serum splits a person's personality into their better and worse natures.

Probably not what you expected, but hopefully still enjoyable!

I frowned at the envelope in front of me, quite confused at the name inscribed on the front. "Holmes," I asked of my friend, who was dedicatedly glueing articles into his commonplace book, "why would your brother be writing me a letter?"

"Brother Mycroft?" I handed him the envelope, and he examined it for himself. "An excellent forgery, if it is one." He gave it back to me, then watched eagerly as I opened it.

"'Dear Dr Watson'…'humbly request your medical opinion on a very sensitive matter…" A phrase caught my eye, and I brought the letter closer, hardly able to comprehend what I was reading. "Jekyll and Hyde?"

"Doctors of your acquaintance, Watson?"

"Of many people's acquaintance, Holmes, though only one's a doctor." Familiar with my friend's distaste for popular fiction, I explained, "Dr Jekyll and Mr Hyde are characters in a novel. The good and bad sides of a man, you might say, or at least that's what seems to be suggested here. Your brother," the words felt ridiculous even as I said them, "has asked me to consult on an antidote to the serum - he believes someone has used it on him."

For a brief moment, I had the rare privilege of seeing Sherlock Holmes completely stunned. Then, with a bark of laughter, he snatched the letter from my hands, poring through it for himself.

As he did so, growing visibly more frustrated as he could not spot any signs of forgery or falsehood, I gathered up my doctor's bag. Whatever the true affliction affecting Mycroft, a patient had asked for my help, and I would not deny it. I would, however, bring along some help.

"Holmes," I asked, a slight grin growing along my lips, "would you care to join me on a case?"