Young Bob Star—the son of hero John Star and Aladoree Anthar, the Keeper of AKKA—lives in the Purple Hall on Phobos, feeling like a prisoner.

Merrin is, in fact a psychopath, but also an incredibly brilliant, former Legionnaire who once subjected Bob Star to horrific torture with the help of the Iron Confessor device developed by the "Reds" of Old Earth.

There, he created a vortex gun based on the plasma weapons of the Medusae and raised a revolt against the Green Hall.

Aladoree Anthar's attempt to destroy the rebels with AKKA failed, as Stephen Orco duplicated the same weapon.

Surviving legionnaires Giles Habibula and Hal Samdu, along with Bob Star, find the wrecked ship of Jay Kalam, which had been destroyed by a Cometeer spaceship not long after leaving Neptune.

The heroes land on a mysterious asteroid - the base of a brilliant scientist (but they also find traces of the Cometeers having preceded them).

Captain Chan Derron of the Legion of Space is given charge of a particularly important task: overseeing the construction of a locked chamber on a remote island, where a famous scientist will field-test his latest invention.

This device has such power and capability that the scientist – Dr. Max Eleroid – fears it himself, and wants to turn it over to the Legion as an adjunct weapon to AKKA, in the defense of humankind.

But when the Legion flagship lands, they find the door unlocked, and Dr. Eleroid dead on the floor, stabbed with the bayonet affixed to Derron's service weapon.

Jay Kalam, Commander of the Legion, agrees to meet with Gaspar Hannas, the casino's owner, along with Hal Samdu and Giles Habibula.

At the casino, a lot is revealed about Giles's past, filling in background that was only hinted at in the previous Legion of Space novels.

Besides being an able cook, an expert geodesic engine man, a legendary lock-picker, and an epicurean, he is a peerless pickpocket, a skill he uses to identify Chan Derron, who is at the casino under a false identity – Charles Derrel.

The Basilisk makes good on his threat: despite the presence of Kalam, Samdu, Giles, and a number of Legion agents, an addicted gambler known as Abel Davian, that night's biggest winner, is snatched from the middle of a police guard and replaced with a ferocious, winged creature of a species no one can recognize.

(Derron had involuntarily become partnered with the strikingly beautiful woman, Vanya Eloyan, whom he suspects to be a wanted fugitive, a female android known as Luroa.)

While Derron is involved with navigation, Giles sends a short message to Jay Kalam, and the Legion begins pursuit.

Meanwhile, the Legion has received terrible news: the Basilisk has whisked away the entire Green Council, all sixty members, along with a number of other important people, including the Keeper of AKKA.

In all, ninety-nine persons are now stranded on a small, rocky island in the middle of an ocean on an inhospitable planet, eighty light-years from Earth.

She also knows the operating principles of her father's stolen invention (she calls it a geofractor), and it explains the Basilisk's ability to transport people and objects at will.

Giles sends a message to the pursuing Legion fleet to back off, that Derron is innocent (they don’t believe him, suspecting he's being tortured).

Their destination is an anomaly Derron had discovered, ten billion miles from the sun and above the plane of the Solar System – he suspects it is connected to the Basilisk.

They locate the hundred missing persons, but cannot remove them immediately: the rocky island, fast being flooded by the rising waters, is shielded from the geofractor's effects.

Derron smashes the small hand-held calculator Davian was using to remotely control a backup geofractor: almost immediately, hostages begin to disappear from the island, as Stella can now reach them.

Davian pours out his bitter motivation: restoration of his family to the glory they enjoyed under the old Empire, and revenge against the casino for ruining him through his gambling addiction.

Derron feels himself being transported back by Stella as the last person rescued from the island, leaving the Basilisk to meet his fate.

[1] P. Schuyler Miller noted that while the novels' "smoothly written space-action plots" were effective, the Giles Habibula character steals the show.

[2] Everett F. Bleiler declared The Cometeers to be "much more elaborate, much more thought-provoking than The Legion of Space, especially with Star's psychological difficulties.