The Battlestar Galactica actress announced last month that she has been cast as "a bad character" in the CBS show's May 16 episode.
Speaking about Helfer's guest appearance, Messer revealed that the Criminal Minds team are forced to step in when her murderous character Izzy Rogers takes hostages during a stick-up.
"Ultimately, she is a serial killer masquerading as a bank robber," the showrunner told TV Guide.
"Our team comes in because the behaviour suggests they do this every month - they kill somebody for the fun of it, not because they're in the way. The woman's off her rocker. She's just a bad egg."
The episode will also see Paget Brewster leave the role of Emily Prentiss for the second time.
Criminal Minds airs Wednesdays at 9/8c on CBS.