Terror of Mechagodzilla (1975)

Just one year after Godzilla's battle with Mechagodzilla, the INTERPOL submarine Akatsuki searches the ocean floor for the wreckage of Mechagodzilla. To the surprise of the sub's captain Kusakari, their scanner detects not a single trace of Space Titanium. As the submarine continues its fruitless search, a whirlpool forms seemingly out of nowhere and brings the Akatsuki to the surface. Soon, a pillar of water erupts from the ocean, and a colossal dinosaur surfaces before roaring and jumping onto the submarine. The creature drags the Akatsuki back to the ocean floor, then crushes it in its hands. The disaster is reported to INTERPOL, which brings in biologist Akira Ichinose to give his opinion on the tape recording salvaged from the sub. To everyone's shock, the tape contains audio of Kusakari exclaiming that he sees a giant dinosaur just before it cuts out. Furthermore, INTERPOL chief Tagawa reports that eyewitnesses saw flying saucers enter and leave the waters where the Akatsuki was destroyed. Agent Jiro Murakoshi is sent to a nearby oceanographic institute along with Ichinose to look into the existence of a giant dinosaur living under the sea. The head of the institute, Professor Ota, tells them about Shinzo Mafune, a brilliant marine biologist who worked at the institute 15 years ago. Mafune was researching ways to construct vast undersea ranches and scientifically control marine life before he claimed to have discovered a huge dinosaur living underwater, which he named Titanosaurus. Mafune said he would gain complete control over the dinosaur, which caused his colleagues to dismiss him as insane and cost him his scientific career. Mafune was fired from the institute, and retreated to his home at Manazuru. After hearing this story, Ichinose and Murakoshi travel to Mafune's home and ring the doorbell several times, receiving no answer. Finally a young woman opens the door and introduces herself as Katsura Mafune, Dr. Mafune's daughter. When the two men ask to speak with Dr. Mafune, she tells them that he died five years ago, and that she burned all of his notes after his death. She apologizes and tells the men she cannot help them, and shuts the door. Both Ichinose and Murakoshi are convinced that Katsura is hiding something as they depart.

Katsura approaches a mirror in her basement, which opens to reveal a secret underground laboratory. Inside, Dr. Mafune is alive and well and toasting with one of his financial backers, Tsuda, to the successful completion of his Titanosaurus control device. Katsura tells her father about the two men who came by, causing him to scoff and declare they came 15 years too late to ask for his opinion. He swears that he will soon unleash Titanosaurus on mankind, showing all those who mocked him that he was right all along and allowing him to have his long-awaited revenge on society. Tsuda tells Mafune that he and the rest of his organization have the perfect instrument for Mafune to complete his revenge, and brings Mafune and Katsura to Mount Amagi. There, he escorts them to a hidden base inside the mountain. There, Mafune witnesses Mechagodzilla being repaired, and realizes that his "financial backers" are actually the Black Hole Planet 3 Aliens. Mafune is greeted by Mugal, the leader of the invasion force, who asks for Mafune's assistance. Mugal asks for Mafune to teach the aliens his control method, then they can unleash both the repaired Mechagodzilla 2 and Titanosaurus on Tokyo. Mafune agrees, and begins helping the aliens perfect Mechagodzilla's control device. Meanwhile, INTERPOL completes construction of the Akatsuki II, and prepares to launch the craft to search for Titanosaurus. Ichinose contacts Katsura and asks her to come along with them, saying it will help vindicate her father's reputation, but she begs him not to go. Ichinose ignores her pleas, and boards the Akatsuki II on its maiden voyage. Tsuda forces Katsura to unleash Titanosaurus against the sub, but before the dinosaur can destroy it, he is incapacitated by the sub's sonar. Titanosaurus writhes in agony, allowing the Akatsuki II to escape. After INTERPOL investigates the incident, they come to the conclusion that Titanosaurus cannot stand supersonic waves, and prepare to weaponize this weakness as a defense against Titanosaurus. Hoping to learn why Titanosaurus reacted the way he did, Katsura contacts Ichinose and gains his confidence by giving him some of her father's notebooks. Ichinose tells Katsura about the sonic wave oscillator they are constructing, and she reports this information to her father. Determined to prove Titanosaurus' strength, Mafune unleashes the dinosaur in Tokyo against Mugal's orders while Katsura and Mafune's butler sabotage the sonic wave oscillator. As Titanosaurus destroys the JSDF units attacking him, it seems Tokyo has no defense against the creature. Tsuda tells Mugal that Mafune could spoil their plans, but Mugal calmly says that Godzilla will arrive and fight Titanosaurus, leaving him too weak to stop Mechagodzilla. True to Mugal's prediction, Godzilla arrives in Tokyo and knocks Titanosaurus over with his atomic breath. The two monsters begin to fight, but Mafune deactivates the Titanosaurus controller after Katsura is shot by INTERPOL and falls off a cliff. Free from Mafune's control, Titanosaurus quickly retreats to the ocean, leaving Godzilla confused.

The Black Hole Planet 3 Aliens' surgeons recover Katsura's body and revive her by repairing the cybernetics they had installed inside her years before after she was injured in a lab accident. Mafune is grateful for the aliens again saving his daughter's life, and promises Mugal he will obey all his commands. When Ichinose learns of Katsura's apparent death, he refuses to believe that she sabotaged the sonic wave oscillator and travels to Mafune's home, where he witnesses a strange vehicle leave. Ichinose follows the car, but is ambushed by a group of armed men, who reveal they had baited him there. Thankfully, Ichinose is rescued by Murakoshi, who shoots several of the assailants and brings Ichinose back to INTERPOL HQ. After Tagawa orders his men to finish repairing the sonic wave oscillator within a day, Murakoshi tells him about the strange men who attacked him and Ichinose near Mt. Amagi. He notes that the silencers on their guns were not normal, and considers the possibility they were aliens. He then received permission to lead a group of men to Mt. Amagi to investigate further. Aware that their location will soon be discovered, Mugal tells Mafune that their base of operations will be moved to Mafune's lab, while the base at Mt. Amagi will self-destruct. He also reveals that the control device for Mechagodzilla has been installed inside Katsura, as Mafune himself claimed Mechagodzilla required living brain tissue to be truly perfect. Mafune is horrified at what he has let happen to his daughter, and breaks down sobbing next to her as she lays motionless on an operating table. When Murakoshi and his men arrive at Mt. Amagi, they witness Mechagodzilla take off from a hangar and find the base abandoned except for several human prisoners, all of whom have had their vocal cords removed. They soon realize the base is rigged to explode, and quickly free the prisoners and escape. Meanwhile, Ichinose returns to Mafune's home to investigate further, but is knocked out by some of the aliens' guards. He is brought inside Mafune's lab, where he comes face-to-face with Mafune himself. He begs Mafune and Katsura to stop, but Mafune says they have come too far to come back now. At Mugal's command, Katsura unleashes both Mechagodzilla and Titanosaurus in Tokyo.

The monsters quickly annihilate entire districts of the city, leaving the JSDF helpless. As the destruction mounts, INTERPOL races to complete the sonic wave oscillator before Tokyo is wiped from existence. Godzilla arrives and attacks Titanosaurus, but finds himself outmatched once Mechagodzilla begins opening fire on him. Together, Mechagodzilla and Titanosaurus overpower Godzilla and knock him unconscious, then bury him in a ravine. Titanosaurus victoriously jumps on Godzilla's earthen grave, but INTERPOL completes their sonic wave oscillator and loads it onto a helicopter. The chopper flies overhead and bombards Titanosaurus with supersonic waves, leaving him helpless. Mechagodzilla prepares to shoot down the chopper, but Godzilla bursts free from the ground and blasts him with his atomic breath. Godzilla engages Mechagodzilla in battle again while Titanosaurus is incapacitated, and quickly gains the upper hand over his doppelganger. Godzilla pummels Mechagodzilla, then tears its head off just as he did before, only to find that Mechagodzilla possesses a mechanical "brain" underneath his head. Mechagodzilla fires a powerful laser from this brain that knocks Godzilla to the ground. As Mechagodzilla repeatedly blasts Godzilla, INTERPOL assembles outside Mafune's lab. Mugal and several of his men leave to deal with them, while Ichinose breaks free of his binds and strangles Tsuda to death. Katsura grabs a gun and prepares to shoot Ichinose, but is shot in the arm by Murakoshi. Mugal returns to the room and opens fire on Murakoshi, then uses Mafune as a human shield when Murakoshi returns fire. Mafune is fatally wounded and Mugal escapes. Mafune's dying words are to scream Katsura's name, which frees her from the aliens' control and restores her humanity. As she lays in Ichinose's arms, Katsura is horrified at what she has done and begins to sob before she realizes Mechagodzilla's controller is still inside her. She begs Ichinose to destroy her, but he says he can't. Finally, Katsura grabs her gun and fatally shoots herself in the chest. Mechagodzilla immediately short-circuits, allowing Godzilla to grab the mechanical monster and throw him into the same ravine where he was buried moments ago. Godzilla fires his atomic breath into the ditch, causing Mechagodzilla to explode and destroying his mechanical double once and for all. Unaware that Titanosaurus is no longer under the aliens' control, Godzilla blasts Titanosaurus with his atomic breath, then grabs him. Mugal evades INTERPOL and boards one of the aliens' flying saucers before they try to escape. Godzilla sees the saucers fly overhead, and destroys them all with a single blast of his atomic breath, putting an end to the Black Hole Planet 3 Aliens. Godzilla lets go of Titanosaurus then blasts him directly in the chest with his atomic breath, causing him to go limp and fall over a cliff into the ocean. Humanity celebrates as Titanosaurus strikes the water and leaves Godzilla victorious. Ichinose sets Katsura's body down on a hill as he watches Godzilla wade back out to sea. He is joined by Murakoshi and the other members of INTERPOL, who pay their respects to Katsura for her sacrifice. Godzilla lets out one more victorious roar before he finally heads out into the open ocean.