Marcie was a high school student at Sunnydale High School. She was not popular, had no friends and was completely ignored, her only real activity being to play the flute. After a while, she became invisible, Giles theorising that it was the result of the Hellmouth manipulating reality on the quantum level; since people essentially perceived Marcie as being invisible, she literally became invisible. Using this new 'power', Marcie started terrorizing the popular students, with Cordelia Chase as her main target. The Slayer, Buffy Summers, found out what had happened to her, but when she tried to confront Marcie she and Cordelia were captured and Giles, Xander and Willow were locked in a basement that Marcie subsequently attempted to 'flood' with gas. Angel rescued the group in the basement due to his lack of a need to breathe, while Buffy managed to defeat Marcie by using her other senses to knock Marcie into a theatre curtain, thus rendering her visible. At the end of the episode, she was taken away by some federal agents, and the last thing we saw of her, she was in a classroom full of invisible students like herself, and a teacher. In the classroom they were going to learn about "Infiltration and Assasination."
Reference to Marcie was made when Buffy turned invisible after being hit by the Trio's Invisibility Ray, with Xander asking if she had been feeling ignored lately and Buffy assuring him that she didn't think this was a "Marcie thing". A year later, after the Seal of Danzalthar was opened and excess Hellmouth energy began to leak out, Buffy saw a young girl begin to fade away, but the girl snapped back into full visibility when Buffy paid attention to her ("Storyteller").
Dark Horse editor Scott Allie was surprised at the popularity of this oneshot character when several fans wrote to his letter column, requesting her inclusion in Buffy the Vampire Slayer Season Eight.