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While Barry and his colleague Julio Mendez investigate the murders of several homeless people whose bodies are being stolen from the crime scenes, Tina reconnects with an old friend, Dr. Carl Tanner, who has recently returned to Central City to retrieve Tina's late husband's research on genetic engineering. Barry and Tina learn that Tanner is using the vagrants in his illegal and lethal experiments to create mutations, and upon being confronted, Tanner injects himself with the unfinished serum, turning into a man monster that only the Flash manages to defeat.