Hope this is satisfactory:
As was often the case, John Watson was greeted by the sound of a rousing argument before he even ascended the steps to 221B.
Since Sherlock had started dating Molly Hooper two months prior, the pathologist had taken to spending almost all her spare time at Baker Street. Neither of them were overly social people (it was part of what made them such a great couple) so there date nights mostly consisted of Chinese takeaway and late night experiments with dangerous chemicals and their various effects on body parts that may or may not have been illegally obtained from the St. Bart’s morgue. Still, they seemed to like it.
The downside to two such intelligent and stubborn people being in a steady relationship, however, was that sooner or later their opinions were bound to clash and it would spark an argument so intense it occasionally prompted Mrs. Hudson to pay John and Mary an impromptu visit until the storm blew over.
The two were in the middle of one such argument when John reached the door to the flat.
"The very foundation of your argument is flawed," Sherlock stated through gritted teeth as he rather carelessly waved a syringe around. "You’re making the assumption that in going back in time, you become a fixed part of the events taking place with no chance of altering the course of events, when, in actuality, every decision you make creates a million different alternate realities in which you don’t make that choice."
"You are so thick-headed sometimes," said Molly, taking the syringe away from him and sticking it into what looked suspiciously like a decaying human foot. "I mean, of course time travel can alter events. That’s inarguable. But certain events can’t be changed. I mean, the universe has to set itself right somehow, right?"
"Second law of thermodynamics, Molly. You’re forgetting the second law."
"I’m not forgetting the second law, numskull, I’m just saying that within that entropy there has to be a point zero or something or else time and space would completely collapse."
"Sometimes I don’t know why I talk to you. Entropy is point zero."
"No, entropy is the chaos that the universe is steadily falling into. You can’t honestly tell me that in each stage of our descent into chaos there’s not a neutral point."
John had had enough. The two lovebirds hadn’t noticed him yet, and if he didn’t intervene they could go at it all night. Normally, this wouldn’t bother him, but it was getting close to his daughter Ella’s bedtime and if they didn’t stop soon Mrs. Hudson would never get to go home, meaning that Ella would be up all night wanting to play with her. So he cleared his throat.
Sherlock and Molly looked up, surprised. Their safety goggles made them look like a pair of odd insects.
"Ah, John, glad you’re here," said Sherlock. "Maybe you can talk some sense into my girlfriend."
Molly rolled her eyes. “He’ll take my side and you know it.”
Sherlock ignored her. “Based on your rather limited knowledge of quantum physics,” he said. “Answer honestly.
"The Doctor should have been able to go back for Amy and Rory, right?"