Jack Drummond is an adventurer who has had enough excitement. He is ready to give it up and return to England to retire. He agrees reluctantly to one more mission, handed to him by his handler, a fellow named Ferguson, to drop off a supply of illegal arms in Tibet. That was dangerous enough but then he agrees to help a boy king and a woman to escape the Chinese and someone destroys his plane and he and his companions are on their own.
Note: there is no proof that this Ferguson is Brigadier Charles Ferguson but he is handling agents for British Intelligence and this is fairly early in his career. Furthermore, there is no proof that he isn't the Brigadier.