A year has passed since the battle between Gamera and the Gyaos, and Japan has struggled to rebuild its cities in the meantime. The military has kept a cautious vigil on the nation's coast, but so far Gamera has yet to return. Then on the night of a meteor shower, while out on field trip Science instructor Midori Honami and her group of kids witness a huge meteor plunges into the mountain snow.
The next night, two security guards are horrified as they see large insect-like creatures stealing glass bottles from a nearby warehouse. Soon after, the entire city of Sapporo is covered with strange plants and the link between these events soon becomes clear. These series of bizarre incidents reveal a new threat to the land of the rising sun.
The meteor has carried with it a species of extraterrestrials. These aliens have set up a hive in the bowels of the city's subway tunnels, deliberately nurturing a plants that grows out of the subway in the city. Soon a gigantic pod erupts from a building and Colonel Watarase of the Self Defense Force realizes that it is dramatically raising the city's oxygen levels.
Just as all hope is lost, Gamera emerges from the sea and heads toward the besieged city. He tears the flower out by its roots, but is ambushed by a swarm of the alien insect soldiers. As Gamera thrashes to rid himself of the attacking insects, a nearby soldier names them "Legion," in reference to something similar about a person possessed by many evil spirits called Legion mentioned in the Holy Bible. Gamera is forced to retreat, just as the monstrous Legion queen bursts out of the ground and flies off to start a second hive. Her wings are damaged by fighter jets but she survives. Again a Legion flower blooms, this time in Sendai, and again Gamera attempts to stop its explosion. He is intercepted, however, by Legion. Legion makes short work of Gamera, impaling him with her sharp legs and blasting him with her horn beam. Sensing the detonation of her pod approaching, Legion leaves Gamera for dead and burrows off. Gamera limps toward the plant, destroying it by knocking it down before it launches its seed into space. The flower explodes and completely annihilates Sendai, seemingly killing Gamera.
Legion marches on to Tokyo with the intention of planting a third and final flower, but Gamera heads her off in the outskirts. She spawns a swarm of insect soldiers against him, but the military manages to draw them off and destroy them with an electric distraction. Gamera and Legion fight all over the suburbs, and the military lobs a few missiles at Legion in aid. Finally, Gamera manages to tear off Legion's horn, and she momentarily collapses in defeat. Suddenly, Legion rises up and fires laser whips from her stump into Gamera. On the verge of defeat, Gamera then looks into the sky and roars, and light begins to shine down upon him. As Legion closes in, Gamera's chest opens up and fires a powerful plasma beam. Legion is hit by the blast and is blown apart.
Gamera glances toward the human onlookers, and then ascends into the morning sky. As they watch Gamera fades into the distance, mankind is unsettled by his power, and trembles lest he should ever view humanity as an enemy.
- Toshiyuki Nagashima as Colonel Watarase
- Miki Mizuno as Midori Honami
- Tamotsu Ishibashi as Hanatani
- Mitsuru Fukikoshi as Obitsu
- Ayako Fujitani as Asagi Kusanagi, a young girl with a spiritual connection to Gamera.
- Yûsuke Kawazu as Nojiri
- Akira Ohashi as Gamera - The film's titular kaiju, Gamera is a giant flying, fireball-breathing turtle that was created thousands of years ago by an advanced civilization to defend the Earth from world-threatening forces.
- Mizuho Yoshida as Legion - The main villain, Legion is a race of extraterrestrials that resemble ants which travel across the galaxy to colonize unsuspecting planets, killing off the native populace in the process.
- Yukijirō Hotaru as Osako
Gamera 2: Attack of Legion was released theatrically in Japan on July 13, 1996 where it was distributed by Toho. It was released directly to video in the United States in 2003 by ADV Films.
The film was the first daikaiju film to win the Nihon SF Taisho Award (the Japanese Nebula Award) in 1996. This decision sparked a fierce debate in the Japanese Science Fiction community, with many critics arguing that it signaled the death of Japanese SF literature.