Without warning, a hostile alien spacecraft assaults a Japanese Moon base, throwing the outpost into panic and alarms. Back on Earth, a boy named Kenichi Kenichi Ishikawa, his father Dr. Yosuke Ishikawa, his friend Helen Wallace and her father, Dr. Tom Wallace, witness the craft’s dramatic descent into the ocean. The scene quickly shifts from awe to danger as the ship suddenly activates a teleportation beacon, pulling the quartet aboard.
Inside the vessel, a human-looking woman appears and explains that she belongs to an alien race known as the Zigrans. To demonstrate their technological prowess, she conjures a massive earthquake that devastates Japan, a bold warning of Zigra’s capabilities. She goes on to share Zigra’s history and its great scientific advances, which, paradoxically, led to the civilization’s own ruin. In searching for a new home, Zigra has now set its sights on Earth.
Earth authorities are summoned and the alien woman is ordered to surrender. Tom, skeptical and quick to judge, calls the intruder insane, yet Zigra’s influence proves powerful as the woman manipulates him and the other two men into a hypnotic trance. Seizing a chance, Kenichi and Helen muster courage and use the ship’s controls to escape, taking their fathers with them as alarms wail in the background.
Gamera, the iconic guardian of the seas, swoops in to investigate the intruder and to safeguard the children. He dives into an underwater pursuit, while military defenses struggle against Zigra’s ship, whose lasers easily outmatch the jets. The alien woman, now on Earth in disguise, tracks Kenichi and Helen and even hops a ride with a Sea World dolphin trainer to return to the facility where operations have pivoted for the moment.
Within Sea World’s labs, scientists uncover a way to disrupt Zigra’s hypnotic hold using sonic waves. The control breaks, revealing a twist: the hypnotized woman is not a true Earthling but Chikako Sugawara, an Earthwoman who had been aboard a Moon rover during the initial attack and was captured by Zigra. With this revelation, Drs. Wallace and Ishikawa deploy a bathysphere in a bid to wake Gamera, only to discover that Kenichi and Helen have secretly stowed away as well.
Zigra presses the attack, threatening to destroy the bathysphere unless Earth capitulates. The UN commander reluctantly concedes to the alien’s demands, and as an electrical storm billows in, Gamera is miraculously revived and retrieves the bathysphere from the sea floor. The final duel commences: Gamera and Zigra collide in a fierce exchange, Zigra’s blade-like dorsal fin leaving a bleeding gash on Gamera’s chest. Gamera counters by pinning Zigra with a boulder, then uses another rock as a makeshift mallet to strike a musical blow on Zigra’s fins—an act that foreshadows the turtle’s triumphant rhythm. With a final, blazing onslaught, Gamera engulfs Zigra in flame, reducing the alien monster to ashes as the sea and shore return to a uneasy, hopeful calm.
