Number the Stars explores the themes of bravery and true friendship. Lowry relies on historical accounts of actual events, her friend Annelise Platt's firsthand account of living in Denmark as a child during World War II, and her own determination and skill as a writer to relate her thematic messages.
In the novel, bravery centers on individuals as well as the heroism of the entire nation of Denmark. Denmark, a small nation, was unable to defend itself against the Germans, so the King surrendered. However, neither the people nor the King gave up. They sank their own naval ships to keep them from the Germans and they smuggled almost seven thousand Jews across the sea to Sweden. Even though the Danes could not fight the Germans as a unified nation, they showed their bravery with individual courageous acts — saving the Jews "one by one."
Annemarie, the protagonist of the Number the Stars, learns the real meaning of bravery. Initially, she is unsure whether she will have the nerve to stand up for what she believes to be right. She thinks that ordinary people aren't called upon to be heroic, so she has nothing to worry about. When she is told that her friend Ellen and Ellen's parents are going to be "relocated" by the Nazi soldiers, she helps them escape from Denmark, risking her life a number of times. Annemarie discovers that ordinary people, like herself, can be brave when they are called to do so. She learns from her Uncle Henrik that "it is much easier to be brave if you do not know everything," that even brave people are frightened, and if you are determined to stand up for what you believe, you don't consider the dangers.
Other individuals who exhibit acts of bravery are Annemarie's parents and sister, Kirsti. Mr. and Mrs. Johansen are confronted with the Nazi soldiers several times. First, in their apartment when they are hiding Ellen, and again when they are on the train going to Gilleleje, and yet again at Uncle Henrik's during the gathering to pay respects to Annemarie's "Great-aunt Birte." They ignore the dangers and do what is necessary to protect Ellen and the other Jews. In her innocent manner, Kirsti is brave. She is fearless when confronted by the Nazi soldiers because she is too young to understand the threat of the political situation. Her obstinacy and talkative nature provide a role model for Annemarie when she is confronted in the woods by the Nazi soldiers. Peter Neilsen is also a courageous character, as was his fiancé, Lise. Both young people were members of the Danish Resistance. They fight injustice and work to get the Jews to safety. Both young people die for what they believe is right. Uncle Henrik, also a member of the Resistance, is a hero, too. Using his boat, he risks his life taking Jews to neutral Sweden.
True friendship is another major theme of Number the Stars. The Johansen family has a close friendship with the Rosen family. When the Rosens tell the Johansens that they have to go into hiding, the Johansens do not hesitate to offer their assistance because, "that's what friends do." The Johansen family risks their lives for their friends.
Despite the Nazi rule forbidding friendship between Jews and Christians, Annemarie and Ellen are best friends. Lowry portrays the characters as physical opposites — Annemarie is lanky and has blonde hair and Ellen is stocky and has dark hair — however, they enjoy each other's company immensely. In the beginning of the novel, when the Nazi soldiers confront the girls for the first time, it is clear that their friendship will be tested. Annemarie first proves her friendship for Ellen by yanking Ellen's Star of David necklace from her neck and hiding it in her hand so the German soldiers won't see it. Later, she risks her life when she takes the envelope that Mr. Rosen dropped to Uncle Henrik aboard his boat. All alone, she takes a path through the woods and is confronted by the Nazi soldiers. By acting like her younger sister, she is able to get the envelope with the handkerchief to Uncle Henrik. The Rosens and other Jews make it to Sweden and safety. Annemarie saves Ellen's life.
At the end of the novel, the war is over and Annemarie decides to wear Ellen's Star of David necklace, symbolizing her friendship with Ellen, until Ellen returns to claim it.
Lowry tells about the heroic acts of the Danes and portrays a deep friendship between Annemarie and Ellen that endures the oppression of the German soldiers to instill hope in readers for a world of human decency.