As a cloud-flecked afternoon sky casts a weird ever-changing pattern of light and shadow on the open range land of Texas far below, a sleek, streamlined Beechcraft biplane, holds a northwesterly course for Lakeville. In the cabin, Tank, Gail Nolan and young Christopher Cross sprawl comfortably in their seats, relaxing after their arduous adventure in South America, while Hop acknowledges a radio message...