Nothing lasts forever, much less a beloved television series. We can all point to the moment when our favorite show began to sink into a sloppy pile of mud plop. Maybe a certain character jumps a shark. Maybe the protagonist goes away to college. Or, maybe, the former level of quality is just unsustainable, and we should just be grateful for what once was. 

Case in point: Star Trek. At least the Original Series. By that third season, things had really diminished, quality-wise. Sure, it was still a thrill seeing the Enterprise crew flying around together sorting things out for the Federation, but some of the magic was gone. While there are definitely some great episodes in that last stretch, the critical consensus is that the third season is the show's worst.

Don't just take our word for it, either. The negative view of the show's last couple dozen episodes is shared by Mr. Spock himself, actor Leonard Nimoy, who voiced his opinion in his 1977 memoir, I Am Not Spock.

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In it, Nimoy expounds upon the possibility of revisiting his former, and best-known, role. His feelings were complicated, as it was a role that was very near and dear to his heart. However, he conceded that there would have to be "some very drastic changes" for him to return as Spock. Mostly, he blamed the waning quality as the main reason he'd think twice before donning the pointy ears again.

"There were two major problem areas. One was the story, the other was character. In the area of story, I felt that our scripts had drifted to the point where they no longer represented the excellent science fiction that we had achieved when we were at our best. For the most part I felt the stories were excuses to get us into and out of danger within the given one hour period. When we were really cooking we had dealt with some very exciting and some very profound and even important ideas, relative to the human condition. For the most part, our third year stories were devoid of ideas."

In defense of those last batch of episodes, the third season of Star Trek was a notoriously tricky situation. Series creator Gene Roddenberry was mostly absent from his once-held position as showrunner, where he'd ensure each episode told a story that fit into the broader web he was weaving. That third season was what secured Star Trek its runs in syndication, but that doesn't mean that everything was up-to-snuff.

"In the area of character," Nimoy continued, "a gradual blurring had taken place. Where there had originally been an opportunity for very distinct characterizations and the character interplay, there was now an erosion which destroyed the clearly defined characters, particularly among the three central figures, Captain Kirk, Mr. Spock and Dr. McCoy. The simple, bottom-line truth was the show was no longer fun to play."