What is the main conflict in The Lost Horizon by James Hilton?

Three former classmates from Oxford University are having drinks and reminiscing about a mutual friend, "Glory Conway." Conway had been a gifted student and athlete in college but never lived up to his promise. Rutherford says that Conway disappeared along with three other passengers in an airplane that was evacuating them from the war in Baskul to Peshawar. The airplane mysteriously vanished, and no one has heard of it or its passengers for months.