Set six months after the events of Gamera, a meteorite collides with the Z Plan rocket transporting the creature to Mars. Now free, Gamera returns to Earth and destroys Kurobe Dam in Japan. At the same time, three mercenaries are sent by Kano, a World War II veteran, to an island in the South Pacific to retrieve a huge opal he once found and hid in a cave. Despite warnings from the local villagers, the trio find and locate the opal, but one dies from a fatal scorpion sting. The second man, Keisuke (Kano's younger brother) is betrayed by his fellow expeditioner Onodera, who uses explosives to trigger a cave-in and trap Keisuke.
En route back to Japan, Onodera accidentally leaves the opal exposed to an infrared light. The heat incubates the opal - actually an egg - and a lizard eventually hatches upon their arrival. Growing to enormous size very quickly, the lizard destroys the ship and Kobe Harbor. Keisuke, having survived the cave-in, awakens in the care of the villagers. He then returns to Japan with a village girl called Karen, who refers to the lizard as Barugon. Barugon wreaks havoc in Osaka with its freeze-gas emitting tongue and stops the launching of the retaliative missiles with a rainbow-like ray emitted from seven spines on the middle of its back. Barugon encounters Gamera and the two battle, with Gamera eventually being frozen solid.
In the meantime, while debating with Kano on how to recover the opal (which he still believes to be aboard the sunken ship), Onodera inadvertently blurts out that he killed his two companions. He then murders both Kano and his wife to cover up the misdeed. After finding Onodera, Keisuke and Karen subdue him and leave him tied up in his home. Keisuke and Karen travel to the Japanese defense ministry and suggest a plan using a huge diamond to lure Barugon into a lake to drown. The plan fails because the diamond's radiation proves to be not enough. Another attempt by irradiating the diamond with additional infrared radiation almost succeeds, until Onodera, having been released and informed of the diamond by his mistress, steals the gem. Both he and the diamond, however, are immediately devoured by Barugon.
Keisuke discovers that the rear view mirrors of a vehicle disintegrated by Barugon's rainbow ray remain intact, so the military devises a plan to reflect its own rainbow emanation back at it with a giant mirror. Barugon is wounded by its own ray, but despite prompting it cannot be goaded into repeating its mistake. Gamera thaws out and attacks Barugon once again. After a destructive battle it drowns Barugon in Lake Biwa, then flies away. Remorseful over the disaster his greed has caused, with his brother now dead, and having found love with Karen, Keisuke decides to make a fresh start on the island where it all began.
- Kojiro Hongo
- Kyoko Enami
- Yuzo Hayakawa
- Takuya Fujioka
- Koji Fujiyama
- Akira Natsuki
- Yoshiro Kitahara
- Ichiro Sugai
- Bontaro Miake
- Jutaro Hojo
- Kazuko Wakamatsu
- Yuka Konno
- After the success of the first Gamera film, Daiei executives decided the property should be given a more prestigious treatment. As well as shifting the tone to appeal more to adults, thirty-plus year veteran Shigeo Tanaka was assigned as director, with the relatively green Noriaki Yuasa being demoted to director of special effects. This tactic ultimately backfired, and Yuasa would direct the remaining Showa era Gamera films as children's entertainment.
- At eighty million yen, the allotted budget was twice as much as was spent on the first Gamera movie.
- As part of the more adult approach, the actresses cast as island natives were originally intended to to appear topless.
- Since the characters otherwise retain their full Japanese names in the English dub, the female lead going by Karen might appear incongruous. However, it is probable that this is the Japanese name which is more commonly romanized as Karin.
- One of the early concepts for Gamera's opponent was a Scandinavian ice giant. This idea would carry over to Barugon's freeze attack.
- It premiered in America on television through American International Television under the title War of the Monsters.