Mary Sutherland dies of a broken heart. She returns to 221b as a vengeful mosquito that gives Holmes a malaria which makes him forget and confuse faces. Right before Mycroft commits his brother to a home, a case similar to Sutherland's arrives and Watson and Mycroft solve (and resolve) it properly. Mary the mosquito bites Sherlock again, reversing the curse, and buzzes off.
Warning: angst