So here are the elements of this accidental masterpiece: at the beginning, Kirk quotes the first sentence of 'Tale of Two Cities': "It was the best of times, it was the worst of times." A Genesis planet, the death of Spock. Life and death, the best and the worst. The movie ends with Spock's self-sacrifice "It is a far far better thing..." etc. (There's even some echo in the Kirk/Spock relationship to Dicken's Darnay and Sidney Carton in the novel, as Meyer points out.) Life has also been renewed for Kirk by the knowledge that he has a son. He has lost a brother, now he has gained a son. The Khan thread: Khan is the Ahab who is killed by trying to kill his nemesis. Spock is the hero who dies saving his ship.

There is also the no-win scenario -a training test that begins the film--- that as a cadet Kirk beat by re-programming the test to allow a solution, but that Spock never experienced. His solution was self-sacrifice. And in doing it, he forced Kirk to face death (the ultimate no win scenario) for the first time, through the death of his friend. The situation that Khan had faced with the death of his wife, but could not assimilate.

So through the creative contributions of many people, and serendipity to say the least, suddenly all the threads---Khan, Spock's death, Kirk's aging and sudden fatherhood, the Genesis planet---fit together like a master design.

There is an additional symmetry. Early on, Spock pronounces the logical ethic: the needs of the many outweigh the needs of the few, or the one. He will repeat it during his death scene, as the reason for his actions.

But in Star Trek III, another set of ethics will mandate the opposite, and it will be the many who sacrifice for the needs of the one. And once the possibility of Spock's return from the dead was entertained, it didn't take long to realize that they had accidentally created the means to do it: the Genesis Planet.

They had made such a good movie that there had to be more.But they'd done more than encourage a sequel: they had made one necessary.