Helgenberger was a cast member of the show for 12 seasons before leaving in January 2012.
She will reprise her role of Catherine Willows and help solve a cold case that has haunted the CSI team for 14 years in the episode to air Oct. 23.
"It's back to the beginning for 'CSI's' 300th episode, when the team investigates a homicide in the home of a reclusive former casino mogul who was a suspect in a similar crime in 2000," a synopsis released Tuesday said.
"The CSI team recalls who they were and where they were in order to solve the cold case."
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