The story begins in the mid-19th century, just after the American Civil War, in Farnham, England. The chief character, Professor Margrave, is an important academic deeply involved in archaeology and the significance of ancient monuments. An old friend, Edward Buchanan – adventurer, ex-soldier and wealthy rancher who had lived overseas for many years – arrives at his house with a rough drawing of writing on a stone plinth known as a stele. He had discovered this stone during his travels in Central America.

The Professor, who had spent most of his academic life studying and endeavouring to interpret the meaning of inscriptions on ancient buildings and ruins, recognises that the writing is of immense significance and launches a search for further examples. The pair enlist the help of Archie Scott, a young American veteran of the Civil War. The Professor also brings his personal assistant of many years, Jim Cross, a former seafarer with an extraordinary talent for getting things done.

Their search leads to a series of journeys and adventures in Central and South America, set against a background of real historical events and characters. The Professor and his companions experience unexplainable events that seem in some way to be triggered by emanations from the stones. Margrave’s party become embroiled in civil strife and a major regional war. They are involved in battles along the great river and even court intrigues while in pursuit of the meaning of the ancient writings. They discover not only writings of an alien origin, but also the existence of a strange species of humans headed by the sinister Dr. Bonpland, who seem destined to become dominant in South America.