After collecting the evidence to a crime scene involving a murdered ATF agent who was working undercover, Calleigh is run off the road into a canal, corrupting her evidence. Since almost none of the evidence is salvageable, the CSIs must work to uncover new, untouched evidence in order to solve the case. Meanwhile, Calleigh's ex-boyfriend, also an undercover ATF agent, offers interesting, yet conflicting, insight into the investigation.