Thorold shakes Lyra awake and tells her that Lord Asriel packed instruments, took Roger, and left. He says that Lord Asriel needed a child to finish his experiment. Lyra falls out of bed with a scream and realizes what she's done: she brought Lord Asriel Roger, whom he needs for an experiment. Thorold helps Lyra dress in her furs and Iorek leaps up when Lyra calls for him. She jumps onto Iorek's back and they race after the sledge tracks with the other bears. They see Lord Asriel ahead and Iorek nervously stops to listen. Lyra hears the Aurora beginning to crackle and a bear yells that he sees witches. A bear throws Lyra forward into the snow and witches' arrows begin to fly toward the bears.
Lyra's desire to save Roger stems as much from her innate sense of loyalty to her friend as it does from what she's learned about morality over the course of her journey. This shows that as she grows up, part of figuring out who she is means that she's going to hone parts of her personality and identity that were already there, rather than just acquiring new information and coming to a new understanding of her world.
Lyra watches the bears fight the witches but then hears Mrs. Coulter's zeppelin approaching with troops. The Bears erect their fire hurler and send coal flying at the zeppelin. The third shot hits the gas bag and it bursts into flame. Witches help the soldiers on board to get off safely, and with the battle now on the ground, Iorek grabs Lyra and they race after Lord Asriel. Iorek promises to find Lee Scoresby and to tell Serafina Pekkala what happened. They stop when they reach a deep chasm with an ice bridge. Sledge tracks run across it, but Iorek refuses to try. Lyra thanks him for his help and he watches her make her way across. The bridge holds until the very end. Once Lyra is across, Iorek returns to his bears.
After Iorek carries Lyra up the mountain, she symbolically gains independence: she lost Lord Asriel and Roger at the bottom of the hill and now, as she crosses the bridge alone, she's going to lose Iorek too. This shows that Lyra is transforming from a child into a young adult thanks to her experiences and what she now knows about Dust and the Magisterium's activities. All of this knowledge, however, means that she also has to go it alone.