Episode 9 Civilization
The crew of the Enterprise discover a planet which can support life. A pre-industrial society exists there but their scans show tech that should not be there. The crew investigates and comes across an alien race which is secretly mining for certain resources and poisoning the population in the process. A fight ensues, the Enterprise chases off the aliens and the Vulcans agree to monitor and protect the planet in the future.
It’s an interesting episode, Earth doesn’t have a prime directive like Starfleet has in the other Star Trek shows. The Enterprise crew still follows the Vulcan’s advice and tries not to culturally contaminate planets that do not have warp capability.
It setup a lot of potential tension for the future with no set rule about aliens visiting other worlds and no prime directive. Ultimately the show never explored this concept any deeper.
“I Like To Play With Toys” Productions®