As Halloween descends on Sin City, the CSIs investigate when a creepy mannequin prop inside a popular haunted house turns out to be a real dead body. Also, Catherine enlists the help of Folsom to search for her friend who disappeared.

After all the guests attending an upscale fundraising soiree for the science department at a local university fall ill, the CSI team gears up to investigate. Also, Catherine enlists Grace’s roommate to help her find clues after Grace disappears.

Catherine’s frustration intensifies when a lead suspect in Grace’s disappearance is found murdered. The team exhausts all their skills to figure out who killed the suspect, and if his death will lead them to Grace.