Godzilla vs. Megalon (1973)

An underground nuclear test on Asuka Island in the Aleutian Islands triggers seismic aftershocks felt as far away as Monster Island, where Godzilla, Anguirus, and Rodan are disturbed as the island begins breaking apart. Godzilla and Anguirus try to flee, but Anguirus falls and is pulled into a fissure, leaving Godzilla roaring defiantly at the rest of the collapsing island. A second seismic event occurs at Lake Kitayama in Japan, where inventor Goro Ibuki enjoys a picnic with his friend Hiroshi Jinkawa as his little brother Rokuro rides on a dolphin boat in the lake. A whirlpool forms in the middle of the lake and nearly pulls Rokuro in before Goro and Jinkawa pull him back to shore with a life rope. The three then watch as the lake completely drains in a matter of moments. As they drive home, they worry that these repeated nuclear tests will cause civilization to sink just like the ancient continents of Mu and Lemuria. When they arrive at Goro's home, they are attacked by an intruder who is fought off by Jinkawa. Jinkawa gives chase in his car, but loses the trespasser after he throws a grenade out the window of his car and sets a field ablaze. Back in Goro's lab, he and Rokuro find it trashed but no sign of anything being stolen. When Jinkawa returns, he finds that a button he pulled off the intruder's coat is the same color as a deposit of red sand left behind in the lab. He goes to get the sand tested while Goro completes work on his advanced new robot. Goro completes his new robot, which he names Jet Jaguar, and activates it. The robot wanders aimlessly through the lab, guided by cameras installed in its eyes. Jinkawa reports to Goro that the sand they found comes from an ancient stratum, confusing them both as to just who the intruder was. While Rokuro rides his minibike not far from Goro's lab, he is abducted by the intruder from earlier and his driver, then used as bait for Goro to open the door. The intruder breaks in once more and knocks Goro, Jinkawa, and Rokuro out with his gun, then contacts Seatopia using equipment in the lab.

His message is received in the advanced underwater kingdom of Seatopia, where Emperor Antonio holds a ritual to awaken their guardian Megalon to make war on the surface-dwellers in retaliation for the damage caused by their nuclear tests. Megalon awakens and burrows his way to the surface, where Jet Jaguar, now under Seatopian control, will guide him. Goro and Rokuro awaken inside a container on a truck, which is being driven to the lake where it is to be dumped. Jinkawa awakens in the lab, where he sees the intruder controlling Jet Jaguar remotely. He asks where the man has taken Goro and Rokuro, but he replies that they will be brought to Seatopia where Goro will create other robots like Jet Jaguar for the defense of the kingdom. Jinkawa escapes his binds and subdues the Seatopian agent, forcing him to reveal that the two are being transported in a container truck. Jinkawa sets off to rescue them, but is tailed by two more Seatopian agents. He manages to lose them in a construction area, and continues to the lake. With Megalon sighted on the surface, the two truck drivers are hesitant to continue approaching the lake, only to be held at gunpoint by the Seatopian agent who hired them. Thinking quickly, they throw him out of the truck and prepare to dump their cargo into a dam. Jinkawa arrives just in time and asks the drivers to help him save Goro and Rokuro, but the two tell him to screw himself and steal his car. Jinkawa mans the truck and tries to get the container clear, only for Megalon to burst through the dam. Timing it perfectly, Jinkawa drops the container onto Megalon's hand, which causes it to go flying into the distance. Jinkawa reunites with Goro and Rokuro and finds they are okay. Goro meets with the JSDF and informs them he can get the robot guiding Megalon under his control. He is flown to Jet Jaguar in a helicopter and uses a remote control device to order the robot to seek Godzilla's help. Jet Jaguar complies and flies to Monster Island. With this turn of events, Antonio contacts the M Space Hunter Nebula Aliens and asks for Gigan's help against Godzilla.

Without his guide, Megalon begins to act erratically and jump around wildly. The JSDF confronts him but is quickly wiped out by his napalm and horn laser. Megalon reaches Tokyo and devastates the metropolis unopposed. Fortunately, Jet Jaguar reaches Godzilla and informs him of the situation, prompting Godzilla to set off for the mainland. Jet Jaguar returns to Goro's lab, which the three have successfully taken back from the Seatopian agent. Jet Jaguar confirms his success in reaching Godzilla before flying away. Goro remarks that Jet Jaguar has seemingly developed a will of his own, and leaves with the others to observe him. Jet Jaguar lands in front of Megalon, who laughs at the diminutive robot before Jet Jaguar grows to Megalon's size right before the kaiju's eyes. Jet Jaguar punches Megalon and begins battling with him to stall his rampage until Godzilla arrives. Jet Jaguar holds his own until Gigan arrives to back up Megalon. The two kaiju combined overwhelm Jet Jaguar, but Godzilla soon arrives to help. Godzilla battles both Gigan and Megalon, allowing time for Jet Jaguar to get back into the fight. Godzilla and Jet Jaguar's combined efforts turn the battle in their favor, prompting Gigan to beat a quick retreat. Jet Jaguar and Godzilla waste no time in beating on Megalon until he finally retreats back to Seatopia. The Seatopians close all fissures to the surface and end their war on the surface world. Victorious, Godzilla and Jet Jaguar shake each other's hands and go their separate ways. Jet Jaguar returns to human size and reunites with his creator, who finds his free will is seemingly gone. But perhaps one day it will return if the world faces another similar threat. The three then return home happily with Jet Jaguar.