1-4: Dr. Trevor Jamieson is stranded on the deadly jungle planet Eristan II with an ezwal, a 3-ton, six-legged, telepathic saurian-like creature that dislikes humans.
Having bailed out of a crashing spaceship, Jamieson and the hostile and contemptuous ezwal - whose goal is to drive human colonists off his homeworld of Carson's Planet - must cross hundreds of miles of jungle to make their way to the ship's wreckage.
Jamieson finds a similar intransigence in the human settlers, all of whom have had family members killed by ezwals, and who are angered by his suggestion that the planet be returned to its native species.
With the aid of counterintelligence agents - who are communicating with him telepathically via the ezwal cub - Diddy manages to lure the Rulls into a building in which they cannot use their organic weapons, and kills them with a gun provided to him.
In compiling his short magazine fiction into novel-length fixups, van Vogt often modified plots or transplanted protagonists to fit originally unrelated works into a single continuity; The War Against the Rull is highly characteristic in this regard, as three of the six constituent stories were (as initially published) wholly unrelated to the Rull series, and only two of the six had originally featured the novel's overall protagonist, Dr. Trevor Jamieson.
The oldest of the stories, 1940's "Repetition," had - in its initial form - contained no intelligent aliens at all, being set on the Jovian moon of Europa before humanity had ventured outside the Solar System, during a period of political tension between Earth and a human-colonized Mars.
(Van Vogt also forgot to remove the original story's references to Europa's colonial cities and mines, creating an inconsistency, as Carson's Satellite had earlier been specified to be uninhabited.)
describe the Rulls as bone-white, muscular, carnivorous worm-like beings who either slither or ambulate with the aid of with numerous suckers (which also serve as mouths), and a feeding habit which involves paralyzing large prey (including humans) before burrowing into a carcass and consuming it from the inside.
The fixup incorporated the Ploian alien from the preceding interpolation into the plot, and expanded the ending, in which Jamieson was promoted to the equivalent of secretary-general of a UN-like Galactic Convention.
Unlike the implacably-hostile Rulls, the Yevds had previously communicated with humans and even exchanged ambassadors, with the two species presently engaged in a relatively low-intensity territorial war.
Incapable of hearing without mechanical assistance, they communicated via modulated light and had an organic, chameleon-like ability to project holographic disguises and fire energy blasts.