Journey to Babel
The Enterprise carries diverse diplomats to a conference on Babel, including Spock's parents, with whom he's not on the best of terms. What's worse, the mission is being mysteriously sabotaged.
This is one of TOS's more complex stories, with family issues for Spock and a conference and conspiracy to keep Kirk busy, with the two of course overlapping. Sarek and Amanda are excellently portrayed by Lenard and Wyatt; their characters resonate strongly despite only appearing here and briefly in two films. Their hand stroking is a tasteful and unique sign of their affection. Spock's feud with his father indeed seems more human than Vulcan, both being stubborn. But the ultimate prize for stubborn perseverance goes to Kirk, who hoists his bleeding self out of the sickbed to take command. He also pulls off some pretty strange moves when fighting the Andorian. Our personal favorite: hurl yourself legs-first against a wall, then land on your back beside the enemy. Spock gets points for figuring out the conspiracy - seconds before Bones knocks him out. Fitting the doctor should get the last word - no one listens normally. Also of note: new alien races (Andorian and Tellerite especially), and the selection of food and drink. In the future, we will all eat colored foam squares.
Spock Saves the Day: Spock interrogates prisoners, runs the ship while the captain is incapacitated, gives his father life-saving blood transfers, and figures out who's behind the sabotage.
Oops: Scotty could have commanded the ship while Spock went straight to sickbay. Also, he could have saved Kirk some embarrassment by telling him the Vulcan delegates' identities before they beamed aboard.
Developments: Spock's parents! Exciting stuff. They're just as we would have expected from previous descriptions, but still compelling. We learn that Sarek, like Spock, practices meditation, and that his son had a pet selhat, who appears in the animated Yesteryear. But not much more about Vulcan is demonstrated, since the three confront seemingly-universal family tensions.