Holmes investigates a case of a very wealthy man who was poisoned. During the case he runs into an old friend, Abigail Spencer, under the name Ann Barker. Abigail turns out to be a former convicted murder, though acquitted of her charges as a young girl. Abigail was beaten by her father and shortly after he was found dead. Abigail and Sherlock, at age 15, exchanged letters and from here is where Sherlock realized who he was.
The case comes to heed and the son of the victim was the one who did it. Though, just like Abigail, he was sexually assaulted by his father so he planned to kill him and frame Abigail. In the end, Abigail takes the fall for the murder to protect him and finally pay for her sins.
Sherlock offers his ear to help the young man cope with what he has been through and to also keep an eye on him.