Fact1: small boat is seen chugging through stormy seas, Fact2: female assistant had baby gargantua in possession for study, Fact3: attack on boat was caused by gargantua, Fact4: people of fishing village see gargantua at same time, Fact5: residents After Gaira attacks Tokyo at are urged open shades to drive him out of city night
Output: As the film opens, a small boat is seen chugging through stormy seas. A giant octopus appears from the ocean and seems bent on killing the sole crew member on deck. Suddenly, the octopus releases the man and retracts its tentacles from the boat. Relieved, the sailor peers out the porthole, only to see Gaira, a large green humanoid creature, fighting the octopus. After easily defeating it, Gaira turns his attention to the boat and sinks it. When the sailor is recovered from the ocean, he tells his tale of the large gargantua Frankenstein in the Japanese version to his doctors, who believe he is in shock and spouting nonsense. The press picks up on the story and interviews Dr. Paul Stewart Russ Tamblyn and his female assistant, Akemi Togawa Kumi Mizuno, who once had a baby gargantua in their possession for study five years prior. Dr. Stewart and Akemi try to dispel the idea that the attack on the boat was caused by the gargantua they knew and studied because it was very gentle while in their care. Stewart postulates that the gargantua he studied would not live in the ocean as it was found in the mountains and probably returned there when it escaped from his laboratory five years ago. Another boat is attacked and the people of a fishing village see the gargantua off the coast at the same time that a mountain guide reports seeing the gargantua in the Japanese Alps. So, Dr. Stewart and Akemi go to visit the mountains and send their assistant, Dr. Majida Kenji Sahara, to look at the evidence in the fishing village. Dr. Majida finds tissue stuck to the side of the fishing boat while Dr. Stewart and Akemi find giant footprints in the snow. In the meantime, Gaira comes ashore and attacks an airport. As he munches on a woman he has pulled from inside a building, the sun appears from behind the clouds. Apparently, the gargantua does not like bright light and runs back to the sea. After Gaira attacks Tokyo at night, the residents are urged to turn on all of their lights and open their shades to drive him out of the city. He begins to retreat to the mountains and is met by the Japanese Self Defense Force, who use giant spotlights and bonfires to corral Gaira into a valley. Although conventional tanks, artillery, and machine guns have little effect on him, a newly constructed weapon — maser cannons — badly injures Gaira. Bloodied and bruised, Gaira falls into the river and appears defeated. Suddenly, another gargantua, brown in appearance, comes to his aid. Sanda, as he is known, pulls Gaira from the river and away from the military. It turns out that Sanda is the gentle gargantua that Dr. Stewart and Akemi have studied years ago. This is confirmed when the scientists encounter Sanda in the mountains and he rescues Akemi from falling to her death, risking his own life and breaking his leg in the process. However, he has become leery of humans after seeing Gairas horrific injuries and quickly vanishes once again. Later, he catches Gaira feasting on some boaters and attempts to kill him to stop the carnage. Unfortunately, he is hesitant about harming his brother and this, along with his broken leg, allows Gaira to overpower him before escaping to the sea. Dr. Stewart attempts to convince the military of Sandas innocence and that blowing them up would simply scatter their cells all over the place, leading to the possibility of thousands of gargantuas, as the monsters can regenerate from even a tiny piece of tissue. The press and military remain skeptical. Gaira reappears in Tokyo, no longer afraid of the city lights, and corners Dr. Stewart and Akemi. Sanda arrives to save them once again and attempts to placate his brother, but Gaira is beyond reason and the confrontation escalates to a violent brawl, causing great destruction in the process. The battle eventually leads out to sea, where the military begins an aerial bombardment. Unfortunately, the bombs disturb a giant underwater volcano, and the two monsters are engulfed in smoke and fire. By the time the volcanic cloud dissipates, both monsters have disappeared without a trace.