Prom night at Nathanson Academy turns tragic when guidance counselor Robert Greggs is found murdered, his face partially melted by hydrofluoric acid. Sid determines the acid killed Greggs, but also notes he took quite a beating before he died. Mac, already under pressure to make an arrest in the case of the taxi cab killer, is similarly harried on this case when he learns Inspector Stanton Gerrard's daughter, Natalie, is a student at Nathanson. The team gathers cell phones from the students, hoping to piece together the scene at the gym between 10:18 pm, the time Greggs was last seen alive, and 10:30pm, when he was found dead. Adam uses Photosynth to reconstruct the scene at the gym in pictures. Marijuana in a fish tank near Greggs' body leads the CSIs to suspect the Prom king and queen, but they insist the plants' presence was just a prank. Mac and Lindsay process Greggs' office, where Mac discovers a box of money and tokens hidden beneath the closet floorboards. The tokens lead Mac and Flack to a Laundromat, there they discover a hidden gambling hall. Mac catches sight of a man wearing Greggs' watch, but the guy claims Greggs gave it to him to pay off a debt, and has an alibi for the night of the murder. While the CSIs work their case, the Taxi Cab killer stalks another victim.
Hawkes discovers pepper spray around the victim's eyes, and he's traced keys found in the victim's hand back to Natalie Gerrard. Inspector Gerrard isn't happy about his daughter being questioned, but Mac reassures him. Mac and Lindsay speak with Natalie, trying to find out why her grades plummeted and she suddenly opted out of college. When Mac asks her if make-up found on Greggs' sleeve is hers, Gerrard storms in and ends the interrogation. Using Photosynth, Stella and Adam catch sight of a picture of Greggs leaving the gym, following someone wearing skull and crossbones cufflinks. Adam locates the boy, Jesse Carver, in another picture. Flack questions him, but the boy says he was only going to get his girlfriend Lacey Pearlman's keys. Before Flack can get further, the boy's father, Wallace, arrives at the station and puts an end to the interrogation. Mac joins Adam in the lab, studying the images from Photosynth and sees Natalie departing at the same time Greggs left with Jesse. Mac and Lindsay question Natalie again with Gerrard present, but Natalie soon asks to speak to Lindsay alone. She tells the CSI that she and Jesse dated and that he got her drunk one night and raped her. She recalls another man being in the room as well. After seeing Jesse with Lacey, she turned to Greggs, hoping to prevent another girl from suffering what she did. Gerrard looks on, distraught. Natalie turns over the dress she was wearing the night she was raped, and semen on it matches Wallace and Jesse Carver--who are not in fact father and son but two adult men. Jesse, who is actually 32, has been posing as a high school student to get access to teenage girls. Mac and Flack apprehend the two men who say little before asking for lawyers. The CSIs leave the interrogation rooms, but rush back when they hear a gunshot ring out. They discover Gerrard, standing over Jesse's dead body, a gun in his hand.