 Searching the whereabout of starship Defiant which vanished into unchartered space, the Enterprise faces an unresponsive, lifeless spaceship which is visual, gleaming light-green, but virtually undetectable to sensors- the Defiant. Beaming down in atmosphered suits, Kirk, Spock McCoy and Chekov find the entire crew dead, they apparently killed each-other. Chekov is the first who gets symptoms, McCoy finds the ship itself is dissolving, and indeed the Enterprise observes the Defiant disappearing from screen, while its own energy is being drained; Kirk fails to beam back before the Defiant vanishes completely. Spock in command soon faces other problems: the mad-making disease clearly starts spreading among the crew, and the alien Tholian civilization sends ships, which soon decide they are enemies, the annihilation of the first one paralyzes the Enterprise, and the next ones demonstrate the ability to weave in between vessels an impenetrable web in space around the Enterprise, and rather fast. Kirk is presumed dead, as usual McCoy blames Spock but a  posthumous  recording by Kirk reconciles them so he can continue looking for a cure. Uhura is the first who sees Kirk shimmering aboard. Time is running out...