Ammonite is the story of Marghe Taishan, an anthropologist and employee of a government agency, the Settlement and Education Councils (SEC). She is sent as an SEC representative to the planet Jeep (Grenchstom's Planet – "Jeep" being the pronunciation of the abbreviation "GP"). Centuries in the past, Jeep was colonized by people from Earth, but contact was lost with the colony, and now the planet is a target for recolonization by the sinister Durallium Company (mostly referred to as "Company"). Some years prior to the beginning of the story, Company sent an expedition of technical and security personnel (the latter called "Mirrors" for the mirrored helmets on their combat armor) to the planet; a short time later, all of the men and many of the women in the expedition died from a virus, also known as Jeep, which was endemic to the planet. As a result, the planet was placed under quarantine, and none of the surviving members of the expedition have been allowed to leave. One of the mysteries of the planet is that there is a "native" population, entirely female and apparently descended from the original colony.

Marghe is sent to Jeep not only to study the native cultures, but also to test a potential vaccine for the Jeep virus. She makes a journey across Jeep, living with several of its indigenous cultures. Shortly after the start of her journey, she is captured and enslaved by the nomadic Echraidhe, one of whose members, Uaithne, believes herself to be the Death Spirit, the chosen representative of the goddess of death destined to bring about an apocalypse. Escaping the Echraidhe and almost dying in the extreme winter, she reaches the quieter village of Ollfoss, where she joins a family and stops taking the vaccine, accepting the virus into her body and truly learning what it is like to be a native (including how the natives are able to conceive children). She learns the mystic discipline of "deepsearch", and eventually becomes a "viajera", or traveling wise woman. Afterward, she is forced to the center of a conflict between the Mirrors and the Echraidhe under the leadership of Uaithne. Marghe stops the conflict, but shortly thereafter Company, believing the vaccine to have been a failure, destroys the space station orbiting Jeep, apparently isolating the people there. However, Marghe and the Mirrors believe that the Company will probably return one day, a return they must prepare for.

Adaptation to life on Jeep appears to be a greater theme of Griffith's novel, as not only Marghe, but other Company personnel, also eventually are forced to settle on Jeep and adapt to the cultures that its prior colonists have created, in order to adjust to the planetary environment.

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• Ammonite
• Ammonite title listing at the Internet Speculative Fiction Database