Jupiter Jones, advanced explorer for the Galactic Colonization Board and misanthropic loner, is forced by low fuel to land his ship, the Mizar, on the distant jungle planet Yogol after he was accidentally space-warped beyond Alpha Centaurus. There he discovers that the native humanoids are ruled by purple-shelled octopods called the Anolyn, who ages ago shambled from the inland sea of Dra Dur and mind-controlled the humans by attaching their snail-like young to the back of the humans' necks, forcing the humans to carry the octopods around the jungle on litters, engage in blood-sports and inhuman orgies, and service their nameless cities from which the Anolyn lord over the world.
Jupiter encounters some wild humans but is soon captured by the Anolyn's mind-controlled forces and fitted with an octopod parasite, allowing the Anolyn to probe his mind. During transport to the city, one of the wild humans who was also captured -- a naked, cat-eyed woman named Lete -- teaches Jupiter their language. Lete believes he's the "Wanderer-from-Beyond" prophesied to free the humans from octopus tyranny. The Anolyn have been breeding domesticated humans for special purposes: laborers, warriors (Nehogan), pets (Caligan), even food stock (the last is particularly troubling to Juipter since his captors have been feeding him meat).
When they reach the city he's separated from Lete, she being sent to the "training pits" and he to the "house of the Radiant God". The God turns out to be a huge decapod statue made of glowing-green radioactive material used by the Anolyn to mutate humans for crossbreeding, but which Jupiter realizes could fuel his ship. While awaiting interrogation, he meets a pet human, a pink-skinned Caligan woman named Tabak (who's also naked, except for a living fur coat thing called a boj). She uses their parasites to telepathically mind-meld with Jupiter, exchanging all their knowledge. Realizing he's the Wanderer, she later conspires to free him.
They retrieve Jupiter's captured equipment, which includes a paralyzing drug, exsrocain, that he uses to remove the parasite from Tabak (and her from him, since she now has all his technical skills). Jupiter re-encounters a Nehogan guard named Reiloc who was skeptical of Lete's "Wanderer-from-Beyond" nonsense before but now believes and joins their new rebellion. The three rescue Lete, but she nearly gets them captured since her parasite lets the Anolyn know where they are and possess her. They remove her parasite and eventually escape the city with a chunk of the Radiant God.
After days of dodging Nehogan hunting parties through the jungle, they reach Lete's people and start organizing an army to take the Anolyn's city. Back at the Mizar, Tabak asks Jupiter to explain what is the emotion called "love" (no, seriously, that actually happens) and warns him that Lete's a hussy, basically. The Radiant God chunk is enough to launch the Mizar and land it violently in the middle of an encampment of Anolyn forces (see illustration), killing the octopod mind-control leaders and convincing the Nehogan warriors that Jupiter Jones is the foretold Wanderer.
In a surprise attack, Tabak and Reiloc are swooped up by the Anolyn's web-birds and carried to the city. The Anolyn send a human messenger with threats to vivisect the pair if Jupiter doesn't surrender. Not really having any choice since the other humans don't care about these two random people, the plans for war continue. Gallons of exsrocain are synthesized and the Nehogan warriors are freed of their parasites.
Jupiter hatches a plan to rescue Tabak and Reiloc (and open the city gate for invasion) by sneaking into the city at night through an underwater passage using a spacesuit, carrying the Mizar's main needle-ray gun (which isn't so heavy for him on the low gravity planet). He succeeds, finding the two, vaporizing some guards and opening the gate. The city falls and the Anolyn retreat back into the nearby Dra Dur sea.
The dénouement finds Lete, now a leader among the united Yogolian humans, planning to take the remaining Anolyn cities; and Jupiter Jones, having shoved the rest of the Radiant God in the Mizar's fuel hoppers, leaving Yogol with stowaways Tabak (who finally admits her "love", as you Earthlings say, for him) and Reiloc (who can't figure out how to eat in zero-g), ready for further adventures on his quest to wander the stars in search of a way home. (As far as I can find there were no other Jupiter Jones stories, so insert your own fan fic here.)
It's no surprise that this story appeared in a pulp magazine. The pulp industry -- both on the lumber side and the publishing side -- was waging a propaganda war against tree octopuses, which they saw as a threat to their businesses through potential habitat protection and the more expensive paper that would bring. This was also part of the larger Weyerhaeuser Conspiracy to rob Sasquatch of their homelands and gain total control over North American forests (for an example of synergy in their propaganda, see: "In The Lair Of The Space Monsters").
McDowell depicts his tree octopus stand-ins as occupiers of a jungle, rightfully belonged to humans, that needed to be driven back into the sea "from whence they had arisen", justifying this by showing them mind-controlling humans into not only slavery, but moral corruption (orgies, cannibalism, lovelessness, etc.). There is also in this a not-so-subtle implication that tree octopus supporters are being manipulated and possibly corrupted, as if every conservationist had an octopus stuck to their neck telling them what to do. In fact, it was humans who were researching octopus mind-control, not the other way around.
While aquatic octopuses would occasionally engage in swarming atacks on humans, usually in response to perceived human aggression from food theft, arboreal octopuses have been nothing but peaceful and undeserving of such scurrilous lies.