Kirk is pulled through a 'corridor' (which they suspect to be like that in The Alternative Factor) into a mirror universe, somewhat different from his own. In place of Spock, he finds T'Pok, a female human-Vulcan hybrid; Scotty is dead, having been killed in an accident; there is apparently the danger of a crewmember seeking a promotion by assassinating his or her superiors. Meanwhile, the Kirk from that universe finds himself in 'our' Kirk's place. Things are--eventually--resolved amicably.
T'Pok is married; her husband, Suré, experiences pon farr during the story. Vulcans--and women?--are apparently not highly regarded in the mirror universe.