Sherlock and John investigate the mystery of Sherlock's missing sibling, and the trail leads to Sherringford.
Sherlock is drawn into the case of Culverton Smith after the man's daughter claims that he wiped her memory after confessing that he was a serial killer. Meanwhile, both Sherlock and John deal with their grief in the aftermath of Mary's death.
While waiting for Moriarty to make good on his threat from beyond the grave, Sherlock distracts himself with cases. He stumbles upon one that he believes is connected to his nemesis.