10 Star Trek Episodes That Paid Off WAY Later
3. The Tholian Web
The Tholian Web hails from The Original Series' oft-maligned third season. The budget for the show had been slashed as it went into its final year, with the initial production of The Tholian Web requiring some creative ideas to make work. A new Constitution-class model was created, rechristened the USS Defiant, and given a green hue for all of its scenes. As well as this, the Tholian web-spinners were introduced.
Both the Defiant and the Tholians would return in the fourth season of Star Trek: Enterprise. Here, it is explained that once the ship vanishes, it reappears in the Mirror Universe. Captured by the mirror-Tholians, they are attempting to strip it for parts. However, the ship is taken by the Terran Empire, leading to some fairly big headaches for the Tholian Assembly.
The two-parter In A Mirror, Darkly is one of the best stories in Enterprise, coming right at the end of its final season. It ties a nice bow on the story of the Defiant, though it would be mentioned again in Star Trek: Discovery. It also goes a way to explaining how the Terran Empire remained as strong as it was for so long, with the infusion of future technology.