While doing research on ways to control sea life, Doctor Mafune uncovers two living dinosaurs called the Titans. Revealing his findings to his colleges, the other scientists disregard his outlandish claims and after an outburst Mafune leaves the institution. However, the doctor continues his research with the help of his only daughter, Katsura. Unfortunately, things go wrong and Katsura is electrocuted in the process. Oddly enough, several mysterious men then burst into the doctor's laboratory and carry off the injured Katsura. The men manage to save the doctor's only daughter, but their miracle comes at the cost of her humanity as she becomes a cyborg.

Many years later the mysterious men return and ask for the aid of the two Titans, which Mafune has learned how to control over the years. The mysterious men reveal themselves to be spacemen from the black hole, and explain how they want the Titans to join forces with their newly constructed Mechagodzilla II. A proposition to which the doctor agrees. However, Mafune starts to become envious of the robotic monster, and sends out the two Titans to attack Tokyo ahead of schedule. The two Titans manage to merge into a much larger Titanosaurus, which has a brief fight with Godzilla before retreating back to the sea after Katsura's life is put in danger.

The aliens, after saving Katsura for a second time, scorn Mafune for his disobedience. Still, a second attack is prepared, this time with both Titanosaurus and Mechagodzilla II. The two creatures immediately start leveling Tokyo, and Godzilla once again appears. During the creatures' skirmish, they manage to destroy all of Tokyo until Godzilla finally manages to claim victory after Titanosaurus is immobilized by Super Sonic Waves, and Katsura, who was turned into a living control unit for Mechagodzilla II, commits suicide shutting down the robotic monster.

The original screenplay that Yukiko Takayama created after winning Toho's story contest for the next establishment in the Godzilla series, which was submitted for approval on October 14th, 1974. The original concept is actually pretty close to the finished version of Terror of Mechagodzilla, with most of the changes being budgetary in nature. The most obvious alteration is that of the removal of the two Titans, which merged to create Titanosaurus in the first draft. It was an interesting concept, although something that was also under explained considering the magnitude of such an occurrence of the creatures merging. Another noticeable change to the script is that of the final battle, which doesn't move to the countryside but instead would have reduced Tokyo to rumble during the ensuing conflict between the three monsters.

Note: No picture or concept art exists for the Titans, and the above design was Toho Kingdom's interpretation of the monsters.