How does an event in the novel The Chrysalids move the plot along to another stage of its development?
One event that really moves the plot along in The Chrysalids is when Anne gets married.
Anne does the math and realizes that she has no one to marry among the telepaths. She is in love with a non-telepathic man, and she decides to marry him.
It might be for years, or for always. I've got Alan -- and you want me to waste years waiting for someone who may never come -- or whom I may hate if he does. (ch 10)
Not soon after, the telepaths have to flee. This is because Anne cannot stand being married to a non-telepathic man, and she confides in him her special ability. When he is not sympathetic, Uncle Axel kills him to protect the others.
Then, one Sunday at dusk, Alan was found dead in the field-path that led to his home, with an arrow through his neck. (ch 10)
Anne’s situation shows a darker side of the telepathic trait. Although it seems like an interesting ability, the telepaths are so close that they really cannot understand non-telepathic people, and no non-telepathic person is going to understand them. This severely limits their chances of finding a mate, and might condemn them to a life of isolation and lonliness.