Francis Nolan is found hovering over the body of her lover, Guy Clarendon, in a London hotel with a pistol in her hand. She's convicted of Guy's murder, despite having no memory of ever being there, nor purchasing the gun that was bought in her name. She insists that she's innocent as Holmes and Watson take on the case to see if she is telling the truth. Along the way, Holmes and Watson learn about Guy Clarendon's dark past.
Source: Wikipedia, "[en.wikipedia.org/wiki/Sherlock_Holmes%3A_Consulting_Detective Sherlock Holmes: Consulting Detective]", available under the CC-BY-SA License.