Lyra feels weak, alone, and frightened. Pan comforts her as she wonders out loud why people do these horrible things to children. When Lyra composes herself, she walks along and admires the brilliant Aurora. Pan flies up and points Lyra in the right direction. He says that Lord Asriel has instruments laid out and that Roger is stuck. Suddenly, the Aurora flickers and Lyra thinks that she can feel the Dust. Roger cries out for Lyra and Lyra runs to him as Pan changes form rapidly in distress. When Lyra reaches the ridge, she sees that Stelmaria has Roger's dæmon in her mouth while Lord Asriel fiddles with wires and batteries and brushes Roger aside.
Feeling the Dust symbolically suggests that this event is very adult for Lyra: she can physically detect that she's out of her element as a child. Stelmaria's participation in Lord Asriel's experiment indicates even more strongly that Lord Asriel is cruel and unfeeling, given that his conscience isn't perturbed by the gross injustice they're committing here.
As Lord Asriel connects wires, the Aurora becomes even brighter. Lyra sees a wire heading into the Aurora and knows that a witch put it there. Lord Asriel beckons to Roger and Lyra shouts for Roger to run. Pan attacks Stelmaria and the two young dæmons fight her. Lyra fights the thick Dust and sees palm trees and boulevards in the sky. She pulls Roger away, but Stelmaria catches Roger's dæmon again. Lyra and Roger start to slip off a cliff and can't stop as Lord Asriel attaches a wire to Roger's dæmon. As Roger is torn from his dæmon by the fall, a bolt of light shoots into the sky and illuminates the city with a tearing sound. Lyra stops falling.
In this instance, Lyra has fulfilled most of her destiny: she unwittingly brought Roger to his death and, with the path to the other universe illuminated, she can move on and fulfill the rest of her fate in the next installment of the series. Even though Lyra's journey makes sense now, the fact that Roger's death is still heartbreaking shows that destiny isn't always easy or good for everyone—and that trying to use one's free will to change it is futile.
Lyra watches the golden monkey and Stelmaria meet and touch each other. Mrs. Coulter steps into Lord Asriel's arms. Lord Asriel invites her to come with him and says that this will mean the end of the Church. Mrs. Coulter is nervous and refuses to come. They argue about Lyra and Lord Asriel kisses Mrs. Coulter, but Lyra thinks the kiss looks cruel. Their dæmons behave strangely; Stelmaria presses her claws into the monkey as the monkey relaxes. Lord Asriel again invites Mrs. Coulter to go with him and says he doesn't care about her lies or her lovers. They kiss again and when they break apart, Lord Asriel says that if she doesn't come, he'll forget her instantly. Mrs. Coulter refuses, and watches Lord Asriel and Stelmaria step into the other world. Then she walks back down the hill, sobbing.
The behavior that Lyra observes between the golden monkey and Stelmaria is likely so foreign to her because it's a consequence of being a sexual adult and attracting Dust, things that Lyra hasn't yet experienced. Lord Asriel's cruelty toward Mrs. Coulter shows again that he's not someone that Lyra should trust. Specifically, when Lyra pegs the kiss as cruel and violent, her role as someone able to interpret the truth suggests that the reader should take this seriously. Lyra's idealized vision of her parents' relationship might be just that: idealized, and not real.
Lyra looks at the city in the sky. The sun from it starts to melt the ice on Roger's clothes. Lyra holds his body and feels betrayed by Lord Asriel. Pan scratches her hand and confirms that Lord Asriel is going to find the source of Dust and destroy it. He says that the Oblation Board, the Church, Bolvanger, and Mrs. Coulter all want to do away with Dust, so Dust must actually be good. Excitedly, Pan says that they believed the adults even when they saw them doing evil things. They stare at each other, and Lyra says that they too could look for Dust and maybe get to the source before Lord Asriel. Lyra and Pan decide that this time, they'll ask every question they can think of and go it alone, even though they're never really alone. Lyra says they still have the alethiometer. She puts Roger's body down and she and Pan step into the sky.
The idea that Dust and original sin might be a good thing is wildly heretical, but Pan also gets at the idea that the ability to choose and gain experience is exactly what makes humans human. Having decided this, Pan and Lyra's fight becomes one for all of humanity and its right to exist in a knowledgeable and free state. Specifically, rejecting what the Magisterium says indicates that going forward, Lyra is going to rely more on herself and what she can do, which will push her closer to maturity, experience, and knowledge.