The story (and the entire series) is told from the point of an unnammed first-person narrator who is 30 years old, very wealthy but crippled, and who knows some eastern arts including "soul casting".
At that point, the author is drawn to a large man's body preserved inside a casket--which he revives to the consternation of the yellow retinue, and the cheers of the jewel-city's nobles.
As he has taken the body of a man preserved for over a hundred years (whose soul was banished by a sorceror), the author has to "relearn" Laonese, the universal language of the planet; he learns that the Jewel-city is known as Phaolon, considered the most splendid city on the planet; that its beautiful ruler is princess Niamh the Fair; that the yellow-clad man was Akhmim ruler of the rival city of Ardha (also known as "yellow city"). He is also "brought up to speed" (the body he took was that of a warrior named Chong The Mighty) on swords, bows, and other weapons.
One day, when Chong and Niamh are out on a hunt for celebrating the Festival of mating zaiph, they are confronted by a huge (somewhat larger than a Bengal Tiger) lizard known as ythid; the lizard is killed by an arrow from Chong's friend Panthon, but the spilled blood causes Niamh and Chong to fall into the web of an elephant-sized spider or xoph. As they escape from it, Niamh is drawn to a flower, which turns out to be a vampire species--but before it can kill her and Chong, the two are rescued by a band of outlaws.
The outlaw band is led by a female, Siona, who falls in (unrequited, as he loves Niamh) love with Chong. Chong makes friends with one of the rescuers, Yurgon--but an enemy of another of the band, weasel-faced Sligon (who manages to find out about the secret of Niamh and Chong).
Later, Sligon reveals the secret to Siona (whose father was banished by Niamh's) and strikes Chong with a poisoned dagger--only to be immediately slain by Siona. Chong aids Niamh in getting to the zaiph pens (to escape) prior to succumbing to the wound and poison--at which point the narrator (on earth) regains consciousness.
Under the Green Star was almost a complete Science Fiction novel in itself, but was followed a year later by When the Green Star Calls.