A Small Miscalculation
Thanks to Sherlock's inability to resist satisfying his curiosity, he's been turned into a small child of about seven (or five, depending on who you're asking) years old. Rather than let Sherlock be taken to a laboratory, John volunteers to look after him. After all, he's had to look after a man-child, how hard can a child-child be?
Poor John. He has no idea.
[Features paternal!Lestrade in small doses and eventual Johnlock after Sherlock grows up again]