For the first time in the history of American TV, an announcer is heard over the closing credits of a network TV program, reassuring viewers that the show (in this case, Star Trek) has been picked up for a second season – and making a direct request that the series’ ardent fans bring their concentrated letter-writing campaign to an end. That campaign has actually been secretly organized by Gene Roddenberry with the help of several prominent SF writers and Star Trek fan Bjo Trimble.
Dot Records releases the Leonard Nimoy‘s album Mr. Spock’s Music From Outer Space, a collection of songs both in and out of character as Spock. Aside from providing material for future Golden Throats albums, this record sparks a feud between Nimoy and Roddenberry; Roddenberry claims co-writing credit on the Star Trek theme (which is featured on the LP) and credit for creating Mr. Spock, and demands – to coin a phrase – a piece of the action. 
The 26th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. John Colicos guest stars in the first Star Trek episode to introduce the Federation’s arch rivals, the Klingons. 
During otherwise typical contract negotiations in preparation for the second season of Star Trek, Leonard Nimoy’s agent demands a significant raise for his client – a raise that could well break the back of the series’ already-struggling budget. In response, Gene Roddenberry, Desilu Studios and the show’s producers play hardball, firmly suggesting that Nimoy be satisfied with a normal raise, and they prepare to recast Spock or create a new Vulcan character to replace him. The show’s casting director suggests numerous potential replacements, including Mark Lenard, Lawrence Montaigne, David Carradine (Kung Fu), and Henry Darrow, among several others – all actors who are felt to have a “Vulcan look.” Nimoy – only later learning what demands his agent has made – negotiates his return to the series, while Montaigne is interestingly cast as Spock’s rival in the second season premiere, Amok Time. Henry Darrow will also don Vulcan ears, but not until Star Trek: The Next Generation’s first season.
The 27th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. The episode has been plagued by both script issues and an unusual but key piece of late re-casting.
The 28th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. Joan Collins guest stars in an episode written by legendary science fiction writer Harlan Ellison; rewrites performed on the script prior to filming later become a major point of contention between Ellison and Roddenberry (and, by extension, between Ellison and Desilu Studios/Paramount).
The 29th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. This episode concludes the series’ first season, and introduces numerous “fascinating” differences between human and Vulcan physiology.
The 30th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC, kicking off the show’s second season with its first-ever glimpse of Spock’s home planet, Vulcan, as well as one of the best-known (and most parodied) fight scenes in the history of television, accompanied by some of the best-known TV soundtrack music ever.
The 31st episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. The Enterprise once again trumps mythology with technology after encountering a powerful entity who gives the Enterprise a big hand.
The 32nd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Captain Kirk continues his winning streak of logically talking sentient computers into self-destructing, impressing Mr. Spock to no end. Scotty becomes the latest senior Enterprise officer to die in the line of duty, and by the end of the episode becomes the latest senior officer to miraculously come back to life.
The 33rd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. This is the first appearance of the “Mirror universe” plotline that recurs through the Deep Space Nine, Enterprise, and Discovery spinoff series, and remains a fan favorite. Barbara Luna guest stars.
The 34th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Once again, the Enterprise crew dismantles a machine maintaining a superstitious belief system on a backward world. David Soul (Starsky & Hutch) guest stars.
The 35th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. In one of the series’ most critically-acclaimed episodes, the Enterprise goes up against a monstrous device designed to devour entire worlds, and Commodore Matt Decker goes nuts.
The 36th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. It’s the only episode of the original series themed to a specific holiday (in this case, Halloween, in which it airs in close proximity), complete with a “giant” black cat, a Macbeth-esque trio of witches, and a script by Psycho novelist Robert Bloch.
The 37th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Roger C. Carmel returns – the only recurring singular villain character of the original series – in the role of interplanetary rogue Harry Mudd.
The 38th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Glenn Corbett guest stars as Zefram Cochrane, the man who invented warp drive (and shouldn’t, in the 23rd century, still be alive), a portrayal very different from what is shown in future Star Trek series and movies.
The 39th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Jane Wyatt and Mark Lenard guest star as Spock’s parents in their first appearance in the Star Trek mythology (in fact, their only appearance prior to the animated series or the movies, despite the characters’ prominence), and Kirk gets into a knife fight trying to salvage interplanetary diplomacy. This episode’s depiction of Vulcan/Andorian unease would become a significant element of the later Star Trek TV spinoff Enterprise.
The 40th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Julie Newmar takes a break from the Batcave to guest star in an episode focusing on the prime directive (and Kirk’s typical willingness to break with that most sacred of Federation laws).
The 41st episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. The crew of the Enterprise contracts a disease that ages them rapidly toward death.
The 42nd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Kirk grapples with a killer alien entity that he first encountered early in his Starfleet career.
The 43rd episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Robert Bloch, writer of the novel that inspired Hitchcock’s Psycho, returns with a script recasting Jack the Ripper as an extraterrestrial entity wreaking havoc down through the ages (with an airdate just before Christmas!).
The 44th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. This early script by writer David Gerrold becomes the best-loved episode of the entire series (according to many a fan poll), introducing the rapidly-reproducing, rapidly-eating, Klingon-sniffing Tribbles to the Star Trek universe. This is their only appearance in the original series, though they reappear in two animated series and Deep Space Nine years later.
The 45th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Kirk, Chekov and Uhura find themselves drafted into playing deadly games for the amusement of unfathomable aliens; naturally, Kirk has a female “drill thrall” and introduces her to his own brand of inter-species relations.
The 46th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Kirk and Spock gang up on a society who has taken a piece of outdated Earth literature and turned it into a blueprint for a blood-soaked society. (As heavy as that sounds, much hilarity ensues.) Vic Tayback (Alice) guest stars.
The 47th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. The Enterprise crew goes up against a psychedelic giant microbe (actually one of the most impressive effects of the original series).
The 48th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Though it will become best remembered for the furry horned creature that attacks Kirk, the episode is actually a surprisingly hawkish (for Star Trek) allegory to the Vietnam War written by Roddenberry himself.
The 49th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. Diana Muldaur, who will later become a one-season regular on Star Trek: The Next Generation in the 1980s, makes the first of two classic Trek guest appearances.
The 50th episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. David Brian (Mr. District Attorney) and Valora Noland guest star.
The 51st episode of Gene Roddenberry’s science fiction series Star Trek premieres on NBC. The Enterprise is hijacked by aliens from another galaxy (one of whom, to Scotty’s delight, can’t hold his liquor).
For the second year running, NBC makes an announcement over the closing credits of Star Trek, reassuring viewers that NBC has picked up the show for a third season. Where the second season’s renewal was brought about by a substantial fan letter-writing campaign (with guidance from Gene Roddenberry), the third season pickup comes after fans stage protest marches outside NBC’s headquarters in Burbank and New York, having gotten word (from Star Trek fan Bjo Trimble, by way of Roddenberry) that the series is likely to be cancelled. The renewal is a double-edged sword, however: in the fall, NBC will move the series to Friday nights, a time slot where it’s virtually guaranteed to be cancelled at the end of its third season.