The Enterprise found the USS Defiant adrift in a region of interspace, lost Kirk to the spatial rift, and escaped a Tholian energy web before recovering him.
In 2268 the Enterprise entered unmapped space to search for the USS Defiant, which had disappeared there three weeks before. The crew found it drifting and fading in and out of view, and the boarding party found that its entire crew had killed each other. The region was going through periods of interphase between universes, which also caused madness in the Enterprise crew. As the Defiant vanished, the transporter failed to retrieve Kirk, and he was lost with the ship.
Spock took command and waited for the next interphase, refusing to leave. The Tholians, led by Commander Loskene, claimed the area as their territory. When Spock stayed, Tholian ships began spinning an energy web around the Enterprise to trap it. McCoy and Spock clashed, and they later found a recorded message from Kirk urging them to cooperate.
Several crew members, including Uhura, saw Kirk as a ghost in his environmental suit. At the right moment, the Enterprise used its power to break free of the web and pulled Kirk back from interphase. The Defiant itself crossed into the mirror universe, as later revealed in Star Trek: Enterprise.