On the night of a full moon in August 2007, Marcy was out and about with her girlfriend, Adrienne, after she was convinced by the latter that she could live a normal life. However, at the Beauty Bar, Marcy began to Change and lose control. Her presence alerted a vampire, who then contacted the warlock Catarina Loss to pass the problem onto them. Catarina then called Magnus Bane, and he went to the bar to respond to the call, alongside the Shadowhunter Alec Lightwood.
During the confusion, Marcy ended up scratching Adrienne and scaring the other mundanes in the bar. Magnus and Alec arrived shortly afterwards and were able to contain Marcy and calm her enough so she could transform back to herself. The sympathetic Shadowhunter then reassured her that he would not to report them to the Clave, much to her delight. Marcy was worrying about Adrienne, horrified that she had hurt her, when she returned, insisting that she was fine. The couple kissed, and Adrienne told Marcy that they would figure things out together so incidents like that would never happen again.
People returned to the bar then, and none gave them a second look, assuming that the werewolf attack was a mere dog problem solved by Magnus and Alec, whom they thought to be dogcatchers.
- The Course of True Love (And First Dates) (first and only appearance)