Kirk and Spock find themselves on a planet where no living thing should exist. In contrast, there seems to be vegetation and a thriving population. There is another surprise. A beautiful young woman played by Jill Ireland, who once had a thing for Spock, is on the planet. Also, on the planet is a sort of flower that when approached, fires spores, causing a sense of utter euphoria in the recipient. Most of the crew are infected with these spores which causes them to loll around, forgetting their duties. Spock, though initially traumatized, falls in with the rest. He picks up his romance with the young woman and announces that he has never been happy before. His deep human side is being exposed. Soon Kirk, who has somehow avoided the spores, is the only one left on the Enterprise. He can't possibly manage the ship by itself and the crew is unwilling to have anything to do with assisting him. He is bewildered but doesn't know what to do. Fortunately, as luck would have it, McCoy has brought some of the flowers on board and Kirk is dispatched by one of them. This leads to the solution to the problem.
In some ways this is quite sad because of Spock's experience with love and the loss of it. The positive is that we get to see Leonard Nimoy stretch himself a bit, not playing the stolid Vulcan all the time.
