A woman tries to frame the man she accused of murdering her sister by shooting herself on a bridge. Sherlock deduces the woman's suicide, but when he meets the suspect during a polygraph test, he believes that the man is guilty of being a serial killer. Investigating other families in connection to the killer, Sherlock and Joan are tipped off to another potential victim's mother, but, upon visiting her, realize that the killer "catfished" them. Sherlock continues to harass the suspect. Sherlock realizes that the man's recording studio has a hidden room and the team catches the killer and saves two of his victims. Gregson stands up for Sherlock within the department.