“Star Trek” began in 1966, and when it debuted — in the era before sprawling franchises — it’s hard to imagine anyone was thinking about stories they’d be telling 15 years later, let alone six decades later. Nevertheless, here we are, and looking back, it’s fun to realize that so much of the show’s deep lore can be traced to single lines that were quickly written on the day of shooting or improvised by the actors. Star Leonard Nimoy is responsible for more than one of these occurrences, having been credited for inventing both the Vulcan salute and the iconic nerve pinch — but that’s not all.

Almost exactly a decade after the show came to a close in 1969, “Star Trek” arrived in movie theaters, with Paramount Pictures eager to capitalize on the sudden success of science fiction thanks to “Star Wars.” By the second movie, though, Nimoy was ready to take off the pointed ears for good and send the role of Spock out the airlock. He was so ready to retire from the franchise that, in order for him to appear in “Star Trek II: The Wrath of Khan,” the star asked that Spock be killed off. Reluctantly, the studio agreed, and in one of the most emotional movie deaths ever filmed, Spock sacrificed himself to save the Enterprise. But then Nimoy reconsidered.

Having had a positive experience filming the 1982 sequel, Nimoy decided he wasn’t quite ready to say goodbye and wanted to leave the option open for a return. And with just one word, Nimoy made sure he had that option, saving Spock from death and adding to “Star Trek” lore in the process.

In Leonard Nimoy’s second memoir, “I Am Spock” — which followed “I Am Not Spock” — the actor wrote about having a change of heart during the production of “Wrath of Khan.” He began to think there was a chance Paramount would film another “Star Trek” film, but didn’t realize “just how open that door might be.” Alongside screenwriter Harve Bennett, the pair hedged their bets and came up with a single line of dialogue that would keep that door wedged open. In fact, it was one word spoken by Spock as he performed the Vulcan mind-meld on an unconscious Dr. McCoy: “Remember.”

Nimoy claimed in his book that he didn’t know exactly what the plan would be. What was it that McCoy was supposed to remember? How would it allow for Spock’s return, if at all? But the pair didn’t worry about that in the moment; they simply performed the scene with this new line and left the logistics for the next movie’s writer and director to deal with. Ironically, that director turned out to be Nimoy.

The actor had long wanted to become a director, even campaigning unsuccessfully for directorial duties on episodes of the “Star Trek” TV show years before. Having happily filmed a death scene, Nimoy finally had leverage, because the studio needed him to return as Mr. Spock. Now, in order to get him, they also had to let him helm the sequel. Lo and behold, “Star Trek III: The Search For Spock” was born, with Nimoy behind the camera.

At the end of “Star Trek II: The Wrath of Khan,” Spock dies from radiation poisoning after entering the warp core to manually repair the ship’s engine in a desperate bid to save them from destruction. But thanks to the improvised line just before his heroic act, there was room to bring the character back from the dead. That resurrection added a key piece of Vulcan mythology to “Star Trek” lore, one with profound implications on the wider universe of stories: the katra — the Vulcan equivalent of a soul.

Though Spock’s “essence” had been referenced in the 1967 episode “Return to Tomorrow,” the Vulcan katra seems to be something very different. In “Star Trek III: The Search for Spock,” it’s explained that Vulcans can transfer their katra to another individual and live on after death, and that’s precisely what Nimoy’s ad-libbed line led to. “Search for Spock” sends Kirk and crew on a rogue mission to reunite Spock’s dead body with the katra hiding deep in the recesses of McCoy’s brain. This new concept forever changed the way we viewed Vulcans, who were now shown to possess a far more spiritual side than previously known.

Eventually, katras were explored further in other “Trek” stories. This includes a key three-part episode of “Star Trek: Enterprise” that reveals that the legendary founder of Vulcan logic had transferred his katra before death and had been hiding out in other people’s bodies for thousands of years. In “Star Trek: Discovery,” it’s even established that katras allow for a form of psychic communication, further expanding on the concept. Not a bad legacy for a single word added at the last minute to a movie script decades earlier.