The film opens up with the launch of the space probe Helios 7 that is set to orbit around Jupiter and return to Earth with data of the planet. Helios 7 is soon intercepted by a strange spore like alien life form known as Yog the Space Amoeba.
4 months later Helios 7 is spotted falling back to Earth in the Southwest Pacific. Helios 7 was spotted by Taro on his flight back to Japan. Taro is a journalist and photographer. Taro is soon recruited by a hotel company to photograph an island called Selgio Island. At first Taro is not interested in taking publicity photos for the Hotel but when he discovers that his friend Dr. Kyouichi will be venturing to the island to research legends of giant monsters and that Selgio Island is very close to where Taro spotted Helios 7, Taro could not help but accept this chance at a great story.
Soon after we see two of the hotel companies employees heading out to do some fishing to gather food for Taro, the doctor and Ayako. The 2 employees were sent ahead of Taro, Dr. Kyouichi and Ayako (a representative of the hotel company). The two men go into the water to fish. While fishing the water gets extremely cold and one of the men spots two blue lights in the water and a giant squid monster rises out of the water and grabs one of the men, while the other manages to escape. The island natives come across the lone survivor and he tells them of the attack. The Witch Doctor who is the leader of the natives believes that monster who he calls Gezora is angry about the presence of the men there to build the hotel and that the hotel workers must leave or the monster will continue to attack until they are gone.
We then cut to Taro, and Ayako they are on their way to the island discussing the possibility of monsters being there when they are interrupted by a mysterious man named Obata. Obata reveals that he will be joining them on Selgio Island to do some research on the natives. Dr. Kyouichi joins in the conversation and reveals the news of the attack that occurred on Selgio Island.
Taro, Ayako, the doctor, and Obata arrive on the island guided by a native named Riko. As they make land fall they crack a joke about giant monsters and Riko gets offended and yells at them. Riko then runs off and the group is left standing there confused as to why the surviving employee isn't there to greet them. The group decides to make a camp on the beach and sets up some beach umbrellas. Some time passes and a Jeep arrives with Riko and the surviving employee. Turns out Riko didn't ditch them in anger he just went to go get the Jeep. The doctor questions the survivor about the attack but the survivor gets angry and tells the doctor not to ask about it.
The group decides to get work right away and start exploring the island. The first spot the group visits on the island is the underground caverns. While in the caverns they discover water that the doctor claims is coming in underground from the ocean. While examining the mass of water in the cave they spot 2 blue lights under the water in the cave, and the survivor of the attack begins freaking out screaming that it is Gezora. The water begins to rise and big wave washes up a watch that belonged to the man that was attacked by Gezora earlier. The survivor gets scarred and runs away leaving everyone else in the cave. Riko chases after the crazed man and manages to jump in the jeep.
The survivor drives back to his hut and starts packing to leave the Island. Riko begins yelling at him telling him that he is crazy for leaving everyone else behind. While frantic survivor is finishing up his packing, Gezora rises from the ocean and attacks his hut. Gezora devours the frantic man, then grabs Riko and tosses him to the side. A colony of bats appears in the sky, Gezora seems agitated by their squeals returns to the ocean to get away from the bats.
The group finally reaches the hut and discovers that it has been destroyed. Ayako spots Riko and they rush over to him. The doctor inspects Riko’s wounds and finds that Riko has frostbite. The doctor is really confused by the frostbite wounds since it should be almost impossible to get frostbite on the hot tropical island of Selgio. The doctor flashes a light in Riko's eyes and Riko lets out a scream of horror. Riko is in some sort of state of shock and is just a shell of the man he was before the attack.
The group takes Riko back to his village to try and patch him up to find out what really happened at the hut, but they are unsuccessful in getting anything out of him, because Riko is still in some sort of PTSD state. We learn Obata's true intensions at the village when Taro reveals that he saw Obata stealing the hotel plans at the sight of the attack. Obata reveals that he was sent by a rival hotel company to steal the plans so that his employer can build a hotel on Selgio first. Taro goes to attack Obata but the doctor stops him and insists that Taro instead help figure out what the cause of the attacks is.
The doctor and Taro then go off scuba diving to look for proof that Gezora really exist. While under water the Taro spots Helios 7. Gezora then appears and chases Taro and the doctor down. It looks bad for the as they fight and struggle in Gezora's icy cold grip. A pod of dolphins soon arrives and Gezora bothered by the dolphin's squeals releases Taro and the doctor. Taro and the doctor manage to get safely ashore but spot Gezora coming to land further down the shore.
Gezora quickly arrives at the native village and begins attacking. The villagers begin to flee the village, Gezora grabs an old lady tosses her through the air and to her death. Then witch doctor kneels down in front of the monster and begins praying for Gezora to leave but he is also thrown but survives. While knocking over the huts Gezora accidentally burns itself and shrieks in pain as it continues to destroy the village. The villagers have nothing but sticks and spears in the village to fight back with and it is not enough to hurt Gezora.
Ayako meets up with Taro and the doctor on top of a cliff overlooking the village and reveals that she noticed the fire hurt Gezora. The villagers soon then also arrive on the cliff with two rifles and containers of gasoline that were left behind on the island by the military during World War 2. Taro and the doctor come up with a plan to soak an area with gasoline and lure Gezora with the two rifles. Taro and the doctor begin shooting Gezora to draw him into their trap. The villagers throw torches at the trap once Gezora is on top of it and it ignites. Gezora screams out in agonizing pain as its body is now covered in burns. Gezora slowly stumbles back to the shore and collapses into the ocean to die. As Gezora is dying we see the space amoeba leave the now lifeless body of Gezora. It was the space amoeba that made this normal squid into a giant monster and have the ability to become cold enough to inflict frostbite wounds.
During the attack Obata attempts to escape with the hotel plans on a raft but is soon attacked by a new monster, Ganimes a giant crab. The doctor and Tarp begin shooting but the Ganimes crab shell is too strong for bullets to pierce. The natives attempt to use leftover grenades from the war but Ganimes still continues to quickly rampage across the island. The monster takes out several villagers and for some reason seems focused on Taro and Ayako. Taro shoots one of Ganimes eyes out but continues to pursue. Taro and Ayako are now at the top of a cliff and cornered by Ganimes. Taro manages to shoot Ganimes in its other eye and uses his camera flash to irate the monsters injured eye. The Ganimes dives to attack Taro but misses and falls off the cliff near a conveniently placed gasoline stock pile. Taro shoots the gasoline cans and causes an explosion that blows Ganimes to pieces.
We then see that Obata managed to survive the attack only being knocked from his raft. The Space Amoeba this time takes over Obata's body. The Space Amoeba begins speaking to the Obata and reveals that it intends to conquer the earth.
We then cut back to Taro and the doctor inspecting Ganimes's remains. After a quick examination that doctor comes to the conclusion that the Helios 7 space probe brought back an alien that wants to conquer the earth. Taro doesn't hesitate quickly believes and agrees with the doctors discovery.
Soon after their realization of an alien invasion Riko's lover shows up and announces that they are to get married that night in hopes of making Riko happy and snapping him out of his state of shock. The ceremony begins in the village and everyone begins dancing around having a great time. Taro decides to take a picture of Riko and the flash from the camera freaks Riko out causing him Riko to run into the woods. Everyone chases after Riko and find that he has somehow snapped out of his shock state. Now that Riko is back to normal again the doctor questions Riko about the first Gezora attack. Riko reveals that the bats appeared and Gezora became irritated by them, the doctor then remembers Gezora fleeing from the dolphins and realizes that sound waves weakens the alien. The doctor and Taro then come up with a plan to capture bat and use them to combat the monster.
The group, along with the villagers begins searching the island's caves for bats and find that a lot of the bats have been killed. They begin searching for bats but keep finding that the bats have been killed. During the search Taro and Ayako come across Kamoebas a giant turtle. Kamoebas chases Taro and Ayako cornering them in a cave. Luckily for them this cave has bats that are still alive and Kamoebas leaves the two.
The Obata soon arrives and the cave learns of the plan to capture the bats. Obata pretends to fall asleep so that everyone leaves him alone. While everyone is eating Obata begins covering the cave in gasoline. The group discovers that Obata has been taken over by the space amoeba and that he now has super strength. The space amoeba reveals its full plan of world domination. Ayako begins pleading for Obata to snap out of the alien's control. We then learn that the space amoeba have spread and taken over another crab. Obata snaps out of the aliens control for a short time and frees the bats from the cave. After the bats are freed Obata runs off into the woods.
The bats begin causing stress to monsters causing the space amoebas to lose control of them. Kamoebas and Ganimes begin returning to their natural animal instincts and start fighting each other instead of attacking the natives. The two monsters fight each other for a while but in the end fall into a volcano. Obata then follows the monsters and jumps into the volcano to insure that there is no longer a Space Amoeba presence on earth.
This movie overall had a very consistent amount of monster action all throughout it which makes this one of my favorite movies. One of the things that bothered me about this movie is that, the first conclusion that Dr. Kyouichi comes up with is that aliens are invading the planet, making animals giant, and controlling them. I would think that one of the first assumptions the Doctor should come up with since the island was occupied during World War 2 by the military would be that the animals mutated from some sort of military chemical, or radiation and not just be like "yeah it all makes sense aliens came back on a crashed probe, they want to kill us, so they turn sea creatures into giant monsters to take over the planet. ALIENS!” I think at that point in the movie Taro should have questioned the doctor’s credibility instead of immediately agreeing. I really liked the design of all the monsters ,Kamoebas vs Gamera would be great to see and an underwater fight between Ganimes and Ebirah would probably be a really fun one to see also. Overall this was a great movie and the last non-Godzilla movie that Ishiro Honda directed. This is definitely a great movie to add to your collection if you can find it at a reasonable price.