Alexander will play C, a pen pal of Sherlock's (Jonny Lee Miller) whom he turns to for help with a case. "She and Sherlock have been engaged in a certain brand correspondence for the past few years," Doherty tells TVGuide.com. "They trade erotic letters. They've never met face-to-face, but they will in the episode in which Jean appears. She works in antiquities, and Sherlock believes she may be able to help with a case that he and Joan [Lucy Liu] are investigating."