Stack was an FBI agent and partner to Robert Gale. He was a bit of a rebel, drinking on duty etc. He was assigned to protect Thomas Crawford, Sr. He encountered Cole. After Crawford Sr. was found murdered, Gale immediately suspected Cole and Railly's involvement and he and Stack went on the hunt for them. They later showed up at the scene of Tommy Jr.'s murder, expressing bewilderment at the aftermath of the temporal paradox.
They found and took Cole and Railly into custody and began interrogating them in secret, with Stack beating up Cole and demanding to know whether he was a Nazi spy. Ramse rescued Cole and Railly knocking Stack unconscious.