A research submarine searching for the wreckage of Mechagodzilla is attacked by a strange underwater monster. Interpol investigates the disaster, and concludes that a discredited scientist named Shinji Mafune may have discovered the underwater monster years before.
Interpol agent Murakoshi and biologist Ichinose visit Mafune’s home and are told by the man’s daughter, Katsura, that he is dead.
Meanwhile, the Simeons have returned. They are about to launch a new campaign to destroy Earth and have enlisted the aid of Dr. Mafune, who is not really dead. Mafune has taken control of the underwater monster―called Titanosaurus―and is helping the aliens reconstruct Mechagodzilla.
Katsura calls on Ichinose and warns him not to go underwater in search of Titanosaurus. In a flashback, we learn that she was killed in a lab accident years before, and the aliens created a cyborg body for her mind to inhabit.
But Katsura cannot help falling in love with the biologist, even though she is the key to controlling Titanosaurus. When she sends the monster to destroy Ichinose’s submarine, the biologist activates the sonar, which sends Titanosaurus fleeing. Now science has a weapon against the monster.
Godzilla soon rises from the ocean and moves to attack Titanosaurus. Katsura is shot when she tries to sabotage Interpol’s sonar weapon. Her body plunges over a cliff and into the sea.
The aliens retrieve her corpse and implant the Mechagodzilla control unit inside her chest. She revives and launches Mechagodzilla and Titanosaurus to destroy Tokyo, where they battle Godzilla.
Katsura, meanwhile, overcomes her alien programming at the cost of her life. As she lies dying, she releases control of the monsters. Godzilla easily defeats Mechagodzilla without alien intelligence to guide it, even as Interpol subdues Titanosaurus with their sonar weapon.
Monsters on Screen
Weapons in screen
- Ishiro Honda
- Yukiko Takayama
- Executive Producer
- Tomoyuki Tanaka
- Katsuhiko Sasaki
- Akira Ifukube
- Special Effects
- Teruyoshi Nakano
- Release Date in Japan