"Sarek" marks the second to last appearance by Mark Lenard as this Vulcan character. A year later he'd reprise the role in "Star Trek VI: The Undiscovered Country" and Lenard would die a few years later. Interestingly, later the character would reappear--with Ben Cross playing him in the reboot of the series, "Star Trek" (2009).
When the ancient ambassador arrives on the Enterprise for some important negotiations, the Captain is surprised at Sarek's handlers. Despite his looking amazingly good for a being over 200, his wife and assistant treat him as if he's very frail and are protecting him from others. Why? At the same time, strange things begin happening on the ship. Crew members start acting VERY emotional towards each other-- with angry outbursts, near fights and worse. Could it be that Sarek is somehow responsible for this? And, if so, how?! And, how can they hold an important conference is Sarek makes those around him enraged?!
All in all, a very sad episode of the series, as you see a beloved character succumbing to the sad effects of aging. Now I am NOT complaining about this--it is good to talk about dementias and other types of geriatric illnesses. But it is still quite sad. Well worth seeing.
By the way, Picard makes a brief comment that made me take notice. He indicated that he'd briefly met Sarek before...at his (Sarek's) son's wedding! Did this mean that Spock marries or that he has a sibling who married? I am sure the real die-hard Trekkies could answer this one.