Fatally injured in the crash of his plane at the Lakeville airport Albert Cross, a noted archaeologist, exacted two promisesfrom Hop and Tank before he died. One to look after his thirteen year old son Christopher and the other to see that a large enveloped reached their intelligence HQ in Washington safely. The boys immediately contacted Major McCall who infromed them tat the envelope contained papers and photographs, which would revolutionize the secret of flight dating back to the ancient civilization of the Mayan Indians. But when the major opened the envelope half the contents were missing.  A strange woman dressed completely in black made a sudden appearance claiming to be Christopher's aunt. Hop grew suspicious and her true identity was uncovered.