There will be many different points of analysis as to what can constitute "important" or "essential" events in Boyne's work. I would say that the first and most important plot development would be the family's move to Auschwitz. This helps to set up much of the drama that is to follow. I think that another important plot development would be the discussion that Bruno and Gretel have about life at the camp. The discussion between both of them about what lies on the other side of the camp is significant in that Bruno begins to learn about how the life that Bruno and his family live is one that is different than what others live at the camp. Obviously, Bruno befriending Shmuel is of vital importance. It is at this point where Bruno begins to realize that his scope of understanding and compassion must be enhanced. Through this, so much more about Bruno is revealed, such as his commitment to Shmuel, as well as his maturation about how values such as friendship have to be honored regardless of the condition. I would say that a final key event would be Bruno accompanying Shmuel on his quest to find his father and both of them walking hand in hand to the gas chamber.