The Disciple is set in the summer of 1939, when 13-year-old Karl arrives on an isolated island in the Baltic Sea to work as lighthouse master Hasselbond's assistant. Hasselbond, however, turns him down because of his young age. Karl struggles desperately to stay on and makes friends with Hasselbond's oppressed son, Gustaf, but their friendship changes to rivalry and hate when Hasselbond starts to favor Karl over his own son.