When a mathematical formula that can virtually unlock any computer program in the world is close to being solved, it's 2 authors are shot to death. Add in a random mugger who happens to see one of the shootings, and a cunning killer who knows how to be in two places at once, and you've got a solid set-up for another case for Holmes & Watson. A bit twisty-turvy, and a tad tough to follow at times (it IS mathematical computations, after all), this was a solid ep.
The B-story about Joan being asked for monetary help from the son of a former patient whose death was caused by Joan was okay but was the springboard for a great set-up for yet another evolution in Sherlock & Jane's relationship.
Sherlock: I thought your plan was quite clever. You demonstrated your goodwill without allowing him to take further advantage. You know, I-I'd quite like to go with you next time. To the cemetery. The mistake that you made, it, uh... changed the course of your life. The man seems to have left quite an impression, as well. I'd just like to pay my respects.
Sherlock continues to admire Jane more & more, and this was another example of that admiration...
And the way that Jonny Lee Miller said the words "invisible ink" while he & Watson were at the morgue was too dang funny for words! At least to me... : )
And am I the only one whose missing Clyde the Tortoise & Sherlock's bees? : )