If this is a story of one person’s journey as well as a history of one horrendous part of World War II, how do the plot and the theme of the book overlap?
In my mind, the question is focused on how the book explores political and personal forms of cruelty. Part of what makes the book so compelling is that it details how political cruelty and personal cruelty overlap with one another at particular moments in historical consciousness. What Wiesel uncovers at the end of the narrative for the reader is that consciousness features an experience of pain from cruelty on both political and personal levels that must be recognized in order for it to be stopped. It is at this point where the plot of Eliezer's story and the themes that result from it overlap. Both are the explorations of the experience of political and personal cruelty.
One of the themes of Night is the presence of death. The death seen in the narrative is not necessarily one of natural causes. Rather, it is one caused by the cruelty of another human being upon another. The death through hangings, the gas chambers, beatings and firing squads are on the same level as people on a train beating an old woman, or a son abandoning his father. Pain and suffering that is result of cruelty is where plot and theme converge. The dehumanization that Wiesel feels was so integral to causing the Holocaust happens personally while existing politically. Silencing of voices transpires in both domains. The plot developments that show these horrific events are reflective of a thematic experience where death due to cruelty on both political and personal levels becomes real, identifiable with distinguishing features. Eliezer's journey through these plot developments caused by personal and political forms of cruelty are representative of themes that similarly embrace both realms in the exploration of pain in the human predicament.