The plot centers on the black-clad Legion of the Winged Serpent, a rogue group of human-like telepathic aliens led by Captain Rameses. From their home planet Alpha in the Orion constellation, they face an imminent doom as their star goes supernova. With the clock ticking, Rameses leads a small flotilla of flying saucers toward Earth to judge whether our world could serve as a new home for their people. Through a series of abductive incursions, they reveal Earth to be a perfect fit, discovering that many of its inhabitants are descendants of transplanted humans. To avoid leaving evidence, they erase their trail with a device that drives the abductees to suicide. Their plan grows darker: a larger version of the device could supposedly turn all of humanity inward, enabling the Alphans to seize control of the planet.
Opposing the invasion is the Intergalactic League of Races, a highly advanced alliance of bald, big-headed aliens from Zeta Reticuli. The League maintains a base on Earth, hidden beneath the ocean in a pyramid-like complex. When Rameses lands at the base posing as a friendly researcher, the League reminds him that Galactic Treaty rules forbid contact with humanity. While touring the facility, he is unsettled by a television broadcast featuring human UFO expert and astronomer Professor Allan Duncan, who treats the abductions as a curiosity and enjoys the local entertainments.
Rameses’ crew sabotages one of the League’s three saucers, which is subsequently shot down as it nears a U.S. Army base. The League dispatches its two remaining saucers to investigate, and as they depart, Rameses and his followers slaughter everyone left in the base and destroy their robots. A League saucer returns to regain control, but its crew is killed in a tense shootout. Rameses then targets the remaining League saucer, but the battle ends with his ship defeated and destroyed. He calls in reinforcements hiding behind the Moon, ordering them to hunt down the surviving League craft and unleashes the “extermination device.” The United States military detects the weapon in orbit, but is powerless to stop the ensuing wave of suicides.
The lone League ship, though damaged, reaches out to Duncan for help. Duncan enlists his friend Malcolm, a computer expert portrayed by Henry Ramer, who repairs the League ship using scavenged parts from downtown Toronto. They are soon discovered after takeoff and are intercepted by a Serpent ship, which they shoot down, sending it plummeting into First Canadian Place. Refueling, they depart Earth in hopes of joining forces with other League vessels. Malcolm’s improvised repairs burn out beyond the Moon, so Duncan’s knowledge of planetary data guides their course using his pocket calculator-like calculations to chart a route toward the outer Solar System.
The League squadron advances, and they join forces to strike at the Legion. Rameses leverages the League base computer to plan increasingly superior tactics and begins dismantling League ships. A rogue robot in the base is only damaged, not destroyed, and it regains control, turning the extermination device against them and steering Rameses’ ships into collisions. With his fleet shattered and the extermination device neutralized, Rameses’ chances dwindle. He learns that his sun has already gone supernova during the conflict and, in a final act of desperation, crashes his vessel into the Moon.
Amid the chaos, the extermination unit passes over Toronto, prompting Betty Duncan to slash her wrists. The League rushes to Duncan’s home and successfully revives her, restoring a fragile balance as Earth narrowly avoids occupation by the Alphans.
