This alien race is breeding two genetic lines to become the ultimate weapon in Arisia's cosmic war with their arch-enemies, the Eddorians.

These tales were inserted into the novel following the serialized release, along with chapters covering members of the Kinnison line in World Wars One, Two, and Three.

This conflict is complicated by the arrival of the technologically superior, extra-Solar, amphibian-like Nevians, resulting in the first interstellar war involving humans.

The Arisians undertake a eugenics project to breed two human genetic lines that are intended to become the ultimate weapon in Arisia's cosmic war with Eddore.

The final part of the book, which was originally published as a magazine story, takes up the tale after civilization has been rebuilt with the covert help of the Arisians.

Humanity has explored the Solar System and formed the Triplanetary League, which consists of an alliance of Earth with the governments of Mars and Venus.

Prior to the start of the main story, humans have set up in-system colonies and fought the first interplanetary war against the Adepts of North Polar Jupiter.

A Triplanetary Patrol agent named Conway Costigan is captured by the Nevians, along with his love interest, Clio Marsden, and an old space-hand and friend, Captain Bradley.

Smith also introduced the idea that the original novel's villain, Gray Roger, had always been secretly Gharlane of Eddore, but disguised in human form.

P. Schuyler Miller, reviewing the 1948 edition, praised the novel as the opening of "an epic which has no parallel in science fiction".

[4] Algis Budrys said that Triplanetary "validated the melding of engineers' dreams and pulp grandiosity, dubbed 'superscience fiction'".

[5] Everett F. Bleiler, however, characterized the novel as "old-fashioned space opera, with a typical romance"; he faulted Smith's modifications to fit the story into his Lensman continuity as "far from convincing".