Young Nestor had been engaged by the United States government to undertake a difficult and dangerous mission to South America, and Gates had been sent on from Washington to post him as to the details of the case.  The boys had waited at the San Francisco hotel three days for the arrival of the Secret Service man, and waited impatiently, as Sam Leroy, who was to be the third member of the party, was anxious for the safety of his aeroplane, the Nelson, in which the trip to "the roof of the world" was to be made.