I would have to say that the one of the most important events from the plot is near the end of the novel when the guard shows up looking for three more Jews to take to the ovens. Rivka is one of the chosen and Chaya (Hannah) decides to take her place. What prompts her is that she has "the memories of Lublin and the shtetl and the camp itself ... she lived, had lived, would live in the future -- she or someone with whom she shared memories. But Rivka only knew now." Hannah's last words to Rivka are, "Run for your future. Run. Run. Run. And remember." Hannah bravely walks towards the doors of the oven telling a story to help calm those she is with -- a very brave last act.
The reason this is important is because it is what ultimately ties the two story lines together. Modern-day Hannah is a typical American teenager who is tired of the old stories told by her older Jewish relatives. She doesn't fully appreciate what suffering and horrors they endured to survive the concentration camps. Rivka, who was saved by Chaya, is actually Hannah's Aunt Eva and Hannah's Jewish name is Chaya as she was named in memory of the brave friend who sacrificed herself for Rivka. Hannah, with her time travel experience now knows, remembers, and appreciates the events of the past and the importance of remembering so that it could never happen again.