Because of [reasons] Sherlock gets transformed into a younger version of himself (mentally and physically).
Donovan ends up spending a lot of time with him. Maybe she's somehow the only one available to take care of the child!Sherlock? Or maybe she wants to study him to spot the early signs of psychopathy that she expects to find in him.
Either way, she comes to the grudging realization that he's actually a ridiculously lovable kid, if rather rude and socially isolated. Maybe he even reminds her of her own younger self in some ways.
Bonus: - Sherlock gets turned back into his normal age eventually, and Donovan's reaction to that is explored. - Donovan feeling protective after the transformation, and even still after he gets turned back into an adult.
TL;DR: Sherlock gets turned into a child. Donovan accidentally befriends him.
Gen. Donovan & Sherlock, friendship. Sherlock gets de-aged and Donovan bonds with him
Donovan ends up spending a lot of time with him. Maybe she's somehow the only one available to take care of the child!Sherlock? Or maybe she wants to study him to spot the early signs of psychopathy that she expects to find in him.
Either way, she comes to the grudging realization that he's actually a ridiculously lovable kid, if rather rude and socially isolated. Maybe he even reminds her of her own younger self in some ways.
Bonus:
- Sherlock gets turned back into his normal age eventually, and Donovan's reaction to that is explored.
- Donovan feeling protective after the transformation, and even still after he gets turned back into an adult.
TL;DR: Sherlock gets turned into a child. Donovan accidentally befriends him.