Synopsis: The mirror-universe Kirk, McCoy, Scotty and Uhura beam back to their Enterprise from "ours", following the events of the televised "Mirror, Mirror". Mirror-Spock chooses to arrest, rather than kill, Mirror-Kirk--despite having been ordered to do so by Command.
Mirror-Kirk is thus allowed a chance to survive and retake the Enterprise. Spock still has control of the Tantalus field, of course, but in the end he must confront Kirk on his own. The result of that conflict will change more than just the leadership of the Enterprise, but the fate of the Mirror-Alpha Quadrant forever.
Notes: This is one of the few stories that details the early development of the Mirror Universe in light of the information given us in Deep Space Nine.