When a young girl is kidnapped and held at ransom, Sherlock notices the kidnappers voice as the “left hand” of Moriarty. As realizing that he is going to have to meet up with Moriarty for information, he reveals to Watson that they’ve been writing letters to each other.
Moriarty is able to convince the Feds that she will help assist in the investigation as long as she can do it outside of her cell. Moriarty teams up with Holmes and Watson much to Holmes’ dismay. It is soon revealed that the young girl that was kidnapped is Moriarty’s daughter. Instead of killing any prison guards during her escape, she just restrains them proving to Sherlock that she has changed.
In the end, Moriarty returns to Federal Custody and her daughter is returned safely. Sherlock keeps the letters from her and confesses to Watson that Moriarty still has an effect on him.