Clara traps three classmates in a science building elevator on the third floor, dousing them with chemicals and preparing to set them ablaze.

The investigation leads to , a chemistry student who survived a traumatic house fire as a child. She believes she is testing her victims' purity through fire.

The team struggles because the perpetrator doesn't fit the standard profile of a male arsonist motivated by power or revenge. Dr. Spencer Reid eventually identifies the motive as —a form of Obsessive-Compulsive Disorder (OCD) involving religious or moral obsession.

The unsub is obsessed with the number three, believing it has divine significance.