What is the climax of the story Number the Stars?

The climax of Lois Lowry's novel Number the Stars features the story's heroine, Annemarie, making the delivery of a package to her uncle. 
However, it might be helpful to provide a bit of background to explain the narrative that leads to the climax. The story opens in Copenhagen with the introduction of two girls, a pair of friends named Annemarie Johansen and Ellen Rosen. Nazis are slowly taking over Europe, which is a big problem for Ellen and her family in particular: they are Jewish, and they are afraid that they will be taken away to a concentration camp. 
Annemarie's family decides to help Ellen's family. Together, they hatch a plan: the Rosens will try to escape with their son, while Ellen will stay behind with the Johansens and pretend to be Lise, Annemarie's long-deceased older sister. When Nazis come to the Johansens' house, they are suspicious of Ellen's identity. As a result, the Johansens flee for Sweden, where they will stay with Uncle Henrik.
The plot gets pretty complicated here: what you need to know is that Henrik and Mr. Johansen stage a fake funeral for a nonexistent aunt. Annemarie does not understand why. But she later learns that Uncle Henrik is involved in smuggling Jewish people to safety. This leads us to the climax: Annemarie finds a packet of information that Mr. Rosen accidentally left behind. Something inside the packet, her mother realizes, is of tremendous value to the Resistance. Mrs. Johansen instructs Annemarie to take a basket, fill it with food, hide the packet inside it, and deliver it as fast as she can to her uncle Henrik.
Annemarie's journey to deliver the packet to Uncle Henrik is the climax of the book. She is intercepted by Nazis, who question her but ultimately let her go. It is an important moment because what she delivers to Uncle Henrik, as the rest of the book reveals, is critical to his mission to help Jewish people to safety.