The case is solved, but they've missed the last train back home, so they've checked into this inn, some hole-in-the-wall in rural Wales. John's staring out the window as the sun sets, marvelling at the sky, the lack of light pollution. Inspired, he turns to Sherlock.
"Come outside with me?"
Sherlock looks up. "Why?"
"Humour me?"
Sherlock scowls, but he closes his laptop and stands, following John out to the rear courtyard of the inn. They're alone, with just the wind and the crickets to keep them company. John lays down in the grass, and after a moment's hesitation, Sherlock joins him.
"You see that star?" Sherlock tracks John's finger to one particularly bright spot. "That's Deneb. Now follow along, you see how that sort of looks like wings? That's the constellation Cygnus."
Sherlock snorts, trying to sound derisive, but John can tell his interest is piqued. He craves knowledge, and John's a good teacher.
"And that one?"
"Very funny, Sherlock. That's an aeroplane."
John smirks, feeling Sherlock's gentle laughter next to him. Every point of contact as they lie there - shoulders, hands, hips - is another star, another point of blinding hot white light. Quietly, gently, John takes Sherlock's hand in his, twining their fingers together as his other hand points out the sharp curve of the Corona Borealis.