'The next few days are predicted to be mostly fine and sunny, with light showers and a possibility of heavier downpours on Sunday evening. The... ' A clap of thunder interrupts the weather forecast being broadcast over the radio in Tesco's, causing John to wish that he hadn't given in to Sherlock's persistent nagging to go and 'retrieve vital supplies' just an hour earlier. He grimaces and reluctantly hands over his cash as slowly as possible in order to prolong his stay in the warm and dry shop, rather than being outside in the mess of thunder and lightning that London has become.
"Fine and sunny my arse," he sighs, the shopkeeper grunting quietly in agreement as he accepts and counts up the small change John's had to turn his pockets out for to pay for the milk. He runs over the conversation in his head as he zips up his flimsy, very not-waterproof jacket and peers out of the doorway at the waterlogged street beyond.
'You need to go out, John.' 'No, I don't.' 'You need to retrieve vital supplies.' 'Milk is not a vital supply, Sherlock.' 'Hot beverages are a necessity in this sort of weather, John, lest you catch hypothermia. You should be aware of this, as a doctor.' 'We're not going to freeze to death, Sherlock.' 'No, you're right. We're not going to freeze to death, because you're going out to get milk.' 'No.' 'But you are. I know you are.' 'Really. How'd you come up with that one?' 'Because you're already doing up your shoelaces. Subconscious action, but obviously you've realized that I'm right. Off you trot.' 'You're an insufferable bastard.' 'It's just milk, John.'
"It's just milk. If it's just a pint of bloody milk, why can't you get it?" John mutters bitterly to himself under his breath as he steps out of the doorway and is immediately pelted with a faceful of Arctic rain and a flash of lightning. His foot lands in a pothole filled with water, and suddenly his lovely, waterproof shoes are brilliant at keeping the water in rather than out. Fantastic. "Oh, that's right. You can't get the milk because I can get it instead. As long as I'm on hand, you don't need to do anything!" His foot emerges from the puddle soaked to the skin, and John makes the decision to take it off and just throw it at Sherlock when he gets back.
The walk back is more than mildly unpleasant, to say the least. The fact that the flat is still five streets away when the heavens really open and a waterfall practically begins to cascade down onto his head really doesn't help matters; and the promise he made to himself that he can throw his wet shoe at his flatmate when he gets back quickly becomes a promise that he can find a cricket bat and beat him to death with it. That is, if he doesn't drown first. The possibility of water-related asphyxiation is enough to make John speed up his pace, even if it does mean that every step he takes sends a splash of dirty puddle water further up onto his jeans, which are already soaked through.
By the time he reaches Baker Street, he really thinks he's gained an understanding of exactly why Sherlock's brother carries an umbrella everywhere. One, to defend himself from sudden downpours of rain; and two, to hit Sherlock repeatedly with until he apologises for sending him out into such foul weather. Which is exactly what John is planning to do as he hastily climbs the stairs, two at a time, and throws open the door to the flat. "Sherlock, do you realize what..." John's angry outburst trails off as he fails to locate Sherlock, seeing instead an empty room, exactly as he left it, minus a certain consulting detective.
"Do I realize what...?" John starts as Sherlock's voice sounds from the corner of the room where he can now see his flatmate, there in plain sight with his eyes closed and his hands pressed together against his lips, obviously deep in thought. As expected.
What John didn't expect was for his flatmate to be lying on the ceiling.