Gamera vs. Viras (1968)

An alien spaceship approaches Earth, its occupants the Virasians intent on conquering the planet for their own. However, the ship is attacked in space by Gamera, who badly damages it by smashing into it with his head and breathing fire inside of it. The ship's commander gives an order for a second ship to be deployed, and warns the rest of his race about Gamera just before the ship explodes.

In Japan, a large gathering of Boy Scouts from both Japan and the United States is taking place. When the scoutmaster Mr. Shimida begins roll call, two of the boys, Masao Nakaya and Jim Crane, are nowhere to be seen. The two boys have snuck off to the nearby Ocean Research Institute, where they enter the unattended miniature research submarine. To play a prank, Masao switches the controls for the sub, after which the two boys return to the scout gathering. Shimida announces that a scientist from the institute, Dr. Dobie, will be allowing each scout to take a ride on his institute's submarine. Shimida and Dobie board the sub first to demonstrate, but they quickly find themselves in trouble with the controls reversed. They manage to return safely to shore, where Masao offers to repair the sub and take it for a ride. Shimida tells Dobie that Masao is a genius with machinery, so the doctor agrees. Masao and Jim take the submarine into the water, accounting for the reversed controls. During their voyage, they observe all of the fish swimming around the sub before witnessing Gamera swim by. They race him with the sub, while Jim takes pictures of the kaiju with his camera. The second Virasian UFO arrives in the area seeking out Gamera, and uses its Super Catch Ray to restrain him along with the sub. The boys tell Gamera they are running out of oxygen, and he lifts the bubble created by the ray to help them escape. With Gamera temporarily immobilized, the Virasians observe his memories to find his weakness. After seeing his first battle with Gyaos, they notice that he went out of his way to save the boy Eiichi from the monster. They determine that Gamera's weakness is his kindness to human children, and set out to exploit this. The ship flies over the scouts gathered on the shore, with Gamera in pursuit. Masao and Jim run after Gamera and wave to him, but find themselves abducted by the UFO's Super Catch Ray. The Virasians contact Gamera and tell him they will kill the two boys if he does not comply with their demands. Gamera obeys their orders and touches down on the beach. They fire a mind control device onto the back of his neck and order him to begin a campaign of destruction across Japan. Gamera first attacks a dam then moves on to Tokyo, where he destroys the Tokyo Tower.

Aboard the ship, Masao and Jim try to think of a way to defeat the aliens and free Gamera. While exploring the ship, they find what appears to be a monster trapped in a cage. They assume the monster was captured like them by the aliens, and promise to free it. Jim uses his lasso to restrain one of the Virasians by the arm, but to his shock the Virasian launches his own disembodied arm at the boys and pins them to a wall, where they are trapped by restraints. The alien reattaches his arm and laughs, leaving the two boys trapped. Masao uses his personal wristwatch device to contact his sister Mariko, who is in a meeting with the JSDF where they are discussing what to do. The boys are informed that the United Nations is deciding whether to surrender to the Virasians or to sacrifice the boys' lives and attack the UFO. The boys say they are willing to sacrifice themselves, but the U.N. decides to surrender instead. The boys free themselves and are contacted by Mr. Shimida, who tells them to apply their earlier prank to the aliens' ship. The boys realize they can reverse the controls on the mind control device and free Gamera from the aliens' control. They sneak into the room with the Virasians, who with Earth's surrender order Gamera to wipe out humanity with no opposition. They then inform the aliens that the monster has escaped, and the Virasians quickly run off to investigate. While they are gone, they reverse parts of the machines that control the mind control device and the Super Catch Ray, then use the ray to beam themselves back to the beach.

Gamera soon arrives at the beach, and the Virasians order him to kill the two escaped boys. Instead, Gamera flies at the UFO and smashes into it, breathing fire inside. The aliens flee to the room with the caged monster, whom they address as the "Boss." The Boss tells them he will kill Gamera himself, but that he will need all their lives. He quikly slices off their heads with his tentacle, and from their decapitated bodies the forms of aliens just like the Boss emerge. Seeing this, Masao tells Jim that the Virasians must have kidnapped other humans like them and taken over their bodies to wear as costumes. The undisguised Virasians then merge with the Boss one-by-one, causing him to grow into the giant monster Viras. Viras challenges Gamera, attacking him with his tentacles. Gamera fights off the beast, who leaps into the water and wraps a tentacle around Gamera's neck, trying to drag him under with him. Gamera pulls Viras out of the water and throws him. Viras molds the top of its head into a sharpened point and lunges at Gamera, only to miss and strike a rock instead. Gamera begins throwing Viras by his tentacles. When Viras tries to stab him again with its head, Gamera throws a rock that becomes lodged on Viras' head. Gamera throws Viras into the sea, then leaps in after him. The monsters fight underwater, with Viras trying to swim away. Gamera grabs onto Viras as he swims and rides him like a jet ski until Viras crashes into the shore and sends Gamera flying. Viras then impales Gamera through the underbelly of his shell with his head, then proceeds to stab him repeatedly. Gamera retracts into his shell and takes flight, with Viras stuck onto him. Gamera flies high into the atmosphere where Viras begins to freeze solid. Gamera continues to spin until Viras is released. Viras plumets through the air and strikes the water below, disintegrating on impact. The boys bid farewell to Gamera as he flies away, while the other scouts and the boys' parents arrive to meet them.