The CSI's find a professional ballerina, Ellen White, hung by a rope after being strangled to death in the Manhattan Conservatory of Dance. Danny analyzes the rope and notes traces of gunpowder from an antique gun that matches another case involving a scarlet-haired Jane Doe who was shot and killed in Hell's Kitchen Restaurant. Sid finds evidence of a new antidepressant, still in the trial phase with the FDA, in Jane Doe's system and traces it to Dr. Carly Emerson, the lone psychiatrist in New York testing the drug. After questioning Emerson, they learn that both victims were her patients. Mac links the victims' names and respective locations of the crime scene and murder weapons together to discern that the killer is playing a game of Clue. Another patient of Emerson's is found killed with a candlestick on the green of a golf driving range. Lindsay discovers a snow leopard's whisker on the body, which leads her to question the city's zookeeper who mentions using a rented Zipcar the night before. The CSI's determine that the whisker stuck to the clothing of the other renter of the car that night, Steve Davis, a former patient of Emerson's who fell in love with her. They head to Davis' apartment and find Emerson tied up in the closet as well as a Clue game board detailing the three previous murders. From the game board, they discern that the next target is Emerson's fiancé, a university professor. They find Davis at the university library just as he had stabbed the professor. Davis reveals the details of each of his killings and his contempt for Emerson for ending their sessions after he confessed his love for her. He explained that Emerson had initially used Clue to bond with him and that he wanted to use the game to make her aware of the murders. Lindsay finds herself the victim of an office prank and is determined to find the culprit. Mac confides in Christine about his aphasia.