Set during the events of "The Solitary Cyclist"

"You've been in a fight," Watson murmured.

Sherlock Holmes raised one eyebrow, a wide smile on his face. "My dear Watson, your own studies into the science of deduction have certainly been paying dividends. You have not looked at me from your paper and you know I was in a fight. Bravo!"

"There is blood on your knuckles where the skin has scraped away from your punches," Watson replied, still not lowering his newspaper. "You haven't been bare knuckle boxing in enough time that your skin has lost its callouses."

"Indeed," Holmes replied. "What else?"

"There's blood on your shirtsleeves. It's not yours, but then again of course it's not. You're a remarkable boxer, and I wouldn't expect you to have lost to any of the men you were likely to encounter on an excursion to the countryside."

"Again, bravo, Watson! It was a very one-sided boxing match, I'm afraid. I took the stance of a gentleman, of course, and fought with dignity and graciousness while he rather fumbled about and attempted some very unsportsmanlike conduct indeed. And so now, my friend, the only question that remains is why you are sore at me, for you haven't lifted your paper to speak, a move quite ungentlemanly and which you would not do if you were not upset. Tell me, you aren't still perturbed about my comment yesterday, are you? Or that I investigated today in your stead? I assure you I meant no offense, and I am in too high of spirits to be quite properly apologetic. Do let's be friends again, Watson, and you may chastise me later."

"I wish," sighed Watson, "that you would have at least asked me to come." He began to fold down his newspaper. "Instead you bounded off and… Holmes!" he exclaimed, finally seeing the split in his friend's lip and the ugly bruise on his forehead. "You didn't tell me you were actually injured in the fight!" he chided him. "Yes, let's be friends again, and I will see to your injuries at once."

"They are of no matter," Holmes said with a laugh and a wave of his hand, "though I will certainly not attempt to stop you from what you must feel is your medical duty."

"You're a good boxer," Watson grumbled as he snapped open his medical bag and applied iodine to a handkerchief. "You don't often come home injured from a fight. I certainly didn't think you'd have injuries inflicted by some random country person."

Holmes laughed again, his smile never disappearing even as blood stained his teeth, giving him a rather demented appearance until he swiped it away with his tongue. "Would you like to hear the story, Watson?"

"Yes, please do tell!" Watson insisted, and so Holmes did.

And when, the next day, Holmes asked a bit dramatically for Watson to please, please accompany him to face the denouement of the case together and how he was oh so invaluable, which was, as it happened, quite prophetic, the doctor laughed and agreed, and once more all was right in Baker Street.

For the prompt from Michael JG Meathook: Bareknuckle boxing