Why are the boys on the island?
This book tells the story of a bunch of kids who are stuck on an unihabitated island in the middle of nowhere. They are from England, and England is at war -- we do not know who they are fighting.
The kids were being flown out of England. The were heading for Australia -- this was being done to protect them from the war. Somewhere along the way, the plane they were flying in crashed onto the island.
In the first chapter, we pick up the action as various of the boys are just finding out if anyone else survived the crash.