Kirk disappears into a spatial interphase while the crew is slowly affected by a mysterious force.
The Enterprise finds the starship Defiant, whose crew have died violently, and the ship is slipping into another dimension. Kirk is lost during the rescue, apparently killed, while the remaining crew begin to see hallucinations, and the Tholians, who are building a web around the ship, press them.
The episode shows Spock and McCoy in conflict as they deal with grief and responsibility. It is remembered for the first appearance of the Tholians and for Kirk's reappearance in a spacesuit as a ghostly figure. The Defiant would be revisited in a later series.