(Plot synopsis from the official Buffy website
As the Spring Fling dance nears, Cordelia's new boyfriend is mysteriously beaten up at school by a baseball bat with no one attached to it. The attacker leaves a cryptic message on the locker, which leads Buffy to believe it wasn't an ordinary monster. Giles suggests it might be an invisible creature or a ghost.
Another attack finds Cordelia's friend Harmony flying backwards down a flight of stairs, with no apparent outside force. After hearing a girl's voice, Buffy pursues the invisible attacker to no avail. Buffy enlists Willow's help in trying to get a list of missing kids perhaps this invisible girl is one of them. Meanwhile, Angel appears at the library and says he's worried about Buffy he knows that something big is about to happen but he doesn't know what.
Shock of shocks: Cordelia is elected the May Queen for the school dance. Buffy is convinced Cordelia is the link between the two mysterious attacks. She finds a girl in Willow's printout, Marcie Ross, who she thinks might be the one. Marcie was in the band, and the invisible girl keeps hiding in the ceiling above the band room. Buffy follows her there to find a nest of sorts where the invisible girl has been hiding. Another attack occurs: Cordelia's teacher is almost suffocated.
Further investigation reveals that Marcie was in everyone's classes, but no one seems to even remember that she existed. Giles says she simply became invisible because she was perceived that way it's all due to physics, helped along by living above a Hellmouth. Marcie's goal seems to be to destroy Cordelia, who is already wigged out by the attacks on people close to her. Cordelia's coronation as May Queen that evening would be the perfect place for Marcie to attack, but Cordelia is none too thrilled about being used as bait. As Cordelia gets ready at school for the dance, Marcie yanks her up through the ceiling. Meanwhile, Giles, Willow and Xander hear some haunting music and try to follow it, hoping to find Marcie. The trio is led to the boiler room where they are overcome by gas fumes. Marcie anesthetizes both Buffy and Cordelia, brings them to the Bronze and ties them up. Marcie, armed with an arsenal of creepy surgical instruments, explains to Cordelia that she intends to give her a face no one will ever forget. She slices Cordelia's cheek, but Buffy manages to free herself and a scuffle ensues. Buffy finally knocks Marcie out, just as a pair of FBI agents arrive to take Marcie away. Angel saves the unconscious gang in the boiler room, but Giles tells Buffy it was a janitor he doesn't want to remind Buffy of her lost love. Cordelia acts grateful to the gang, but becomes her normal obnoxious self when her newly-healed boyfriend and her cooler-than-thou friends show up.