Four children and a nanny go missing. John thinks it's a tragedy, Sherlock thinks it's interesting, and Lestrade thinks it sounds like the beginning of a bad joke.
They take the case.
Three days and four nights of sleep-deprived, hunger-driven puzzle solving later, they find the victims safe and sound in a small cottage in Essex. The best part is that John doesn't even have to shoot anyone.
The cab ride home back to London is tense. The otherwise beautiful day blinds John as if he's staring into the sun, and the simple act of breathing makes him exhausted. Sherlock is doing far worse, because he hasn't eaten a single atom in four days. He can feel his stomach grumble and it distracts him, so naturally, he yells at his stomach to shut up.
John yawns and tries to be a good doctor/friend to his demented flatmate. "You really should eat... more..." John feels the world swirl around him into a relaxing sea of darkness as he blacks out.
He wakes up three hours later to find his head on Sherlock's lap, Sherlock's fingers curled in his hair, Sherlock fast asleep, and the cab parked impatiently outside of Baker Street. The cab driver coughs and Sherlock pays him for the long ride before waking Sherlock up.
(John never speaks of it again, but he thinks about it quite a bit.)