It takes learning about grief to teach Lily about friendship. At the beginning of the novel, Lily, her grandmother and her father are headed away from the coast to escape danger from the German navy. Soon after, though, Lily's father joins the war effort by helping with rebuilding areas destroyed by the war. Lily goes through the grief of losing her father's presence and also experiences the fear that her father might not make it home from the end of the conflict.
At the same time, Lily's best friend, Margaret, is headed across the country to Detroit; her father, instead of joining the military, is headed to work in one of the factories that will turn out tanks and other machinery for the war effort. So Lily is left alone, grieving for her father and her friend.
Enter Albert: fresh off the boat from Europe, having escaped the horrors of Nazi occupation. His parents and grandparents were left behind; the only one who came with him was his sister, Ruth. However, she was pulled off the boat while Albert was asleep, just before the boat left, because she had the measles. Albert awakens headed to a new world, safe from the Germans, but alone, grieving not only the people he knew he had lost, but also the sister he had thought would be saved.
As Lily and Albert become friends, they talk about each other's grief. Lily uses Albert's grief to get him to agree to help her get to Europe, but the grief that comes with almost losing Albert, when her plan almost kills him, ends up teaching Lily what it means to be a genuine person and an honest friend.