 Beamed up during an ion storm, which causes a transporter malfunction, the landing party of Kirk, McCoy, Scotty and Uhura find themselves in a mirror universe aboard a parallel Enterprise run by ruthless barbarians. The ion storm also caused their malicious counterparts to beam to the real starship. Kirk and the others must find a way home before they are discovered and exposed by their parallel crew members, who use treachery, back-stabbing and seduction to get what they want.