Most of the events of the novel are set in a fictitious mountainous oasis in the Sahara Desert of Nubia, which is inhabited by a culture preserving traditions from the ancient Egyptian and Meroitic civilizations.

[1] The story begins in August 1907, ten years after the Emersons' expedition into the Nubian desert in The Last Camel Died at Noon.

On that occasion, they had responded to a request from a mysterious mean called Tarek, who had travelled to England to ask them to rescue Mr Forth, a former acquaintance of Professor Emerson.

Their travels had taken them to a Lost Oasis in the Western Desert of Nubia, where a Meroitic – ancient Egyptian civilization had survived for centuries without outside contact outside.

The Emersons had fought to help Tarek claim his ancestral throne, and at the end of the novel had brought Nefret to live with them in England, as her father had wished.

At the beginning of Guardian of the Horizon, a messenger from the Lost Oasis appears at their home in Kent, pleading for help for their friend, King Tarek.

Unlike their first trip, they bring a larger force, including Selim and Daoud, in full awareness that the Lost Oasis is no longer a secret.

These include a British army captain (Moroney), an archaeologist (MacFerguson) and an adventurer (Newbold), who has in his company a young woman, Daria.

Upon their arrival, they find that King Tarek has been deposed by Zekare, the father of the duplicitous Merasen who brought them back.

When Amelia catches an intruder in their quarters, she is relieved to find that it is her enemy and admirer Sethos, disguised as the archaeologist MacFerguson; she persuades him to rescue Nefret.

Leaving Tarek, descending a steep slope, Ramses is wounded, falls a long way down, and is imprisoned by the gleeful Merasen.

Ramses shares his cell with Moroney, former English army captain who had originally saved Merasen from slavers on his outward journey from the oasis.

Publishers Weekly found this novel, where the Emersons return to the Lost Oasis featured in The Last Camel Died at Noon ten years later, to be a good addition to the series.

The overall judgment was that "Peters's knowledge of ancient Egypt ... in the region allow her to dress her melodrama with authentic trappings that add greatly to the enjoyment.

In a brief remark about the novel, their comment was "Kidnapping, murder, political intrigues, and damsels in distress make this sixteenth episode in the Peabody Chronicles every bit as exciting as the others.