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When a dead serial killer arises, it takes Sherlock and Joan on a trail of eight possible suspects all with the same set of bite marks. Our duo discovers that the dead killer, Andrew's teeth was a mold for dentures, thus having the exact bite mark.
Joan has a connection with the murders as she was apart of the operating team who lost Andrew in the middle of surgery. She dealt with guilt that there was more she could have done to save him thinking that he was innocent.
However, it is proven that he was indeed guilty and his mother was using a set of his dentures to try to prove his innocence.