Hop Harrigan, Tank Tinker, and Hop's Aunt Agatha were together in a hotel room in San Rio, a sea port on the western coast of Costa Rica, when a uniformed messenger arrived with a note for Hop. The note was signed by the commander of the Costa Rican airforce, and requested an immediate interview on a matter of great importance. Suspicious, because the handwriting in the note seemed to resemble German script, Hop requested Tank and Aunt Agatha follow him...