Mary & John are married government assassins gone rogue who are perfectly happy with their murderous lifestyle, thanks. Sherlock is the secret weapon the British Government calls in when Mary & John get a little too good at their job and can't be caught.

Mycroft counted on Sherlock getting bored and going back to his pedestrian homicides. He didn't count on Sherlock becoming fascinated and infatuated with his quarry. Nor did he expect that fascination to be reciprocated.

Mary & John love a good chase and god do they love clever. Sherlock is more than happy to oblige them.