TOS #26: Errand of Mercy

The Enterprise gets attacked by Klingons about 30 seconds in to this episode, and then continues on its mission to secure a border-area planet of some strategic importance.

The council of the planet, which was supposed to be a backward one, isn't interested in Federation protection, and keeps insisting it's safe. Then, members of the council start intuiting things about what are happening in orbit.

Kirk takes the pseudonym Barrona and Spock pretends to be a Vulcan trader. Kirk is then appointed liaison between the Klingon marshall and the Organians. He and Spock decide to try to foment a rebellion, but the Organians don't care and the Klingons find out and imprison Kirk and Spock. Eventually, both the Klingons and humans realize the Organians are far more advanced than was thought (it turns out they are godlike beings of pure energy).

Taught a lesson in humility, they call off the war.

TOS #25: The Devil in the Dark

This episode is about a monster menacing a mining colony. It turns out to be a silicon-based lifeform that's only killing to protect its brood. This guy and about 50 other people get killed by it.

I could have told them at this point that this shiny orb was the egg of a silicon-based lifeform, but that somehow eludes everyone but Spock.

Eventually Spock mind melds with the creature, figures out what's going on, and brokers a deal. If the creatures dig tunnels (which is what they do anyway), the humans will NOT dig tunnels and will satisfy themselves with processing the discarded minerals. Everybody wins.

TOS #24: This Side of Paradise

This episode is about the Garden of Eden. In it, all these very healthy-looking guys live blissfully in this Smurf-like one-woman colony. This woman actually loved Spock 6 years ago, and uses these weird bliss-making spores to make him decide to stay in Eden.

Eventually, the whole crew submits to the spores, falling into soma-like trances, until the Captain figures out that strong emotion breaks their hold on the mind. He pisses Spock off enough that Spock can overcome the Spores, and then they all agree not to court-martial each other.

Spock is acutely aware that he gives up his only chance of happiness, but that's OK, because being blissful yet having nothing to live for is no good, says Captain Kirk.