During World War II, the heart of Frankenstein's monster is transported from Germany to Japan, where it arrives in Hiroshima. On the day that it arrives, however, the United States drops the atomic bomb on the city. Fifteen years later, a vagrant child is found by doctors at a hospital in Hiroshima. The child soon draws interest when he begins growing, and soon becomes larger than a car. Eventually, the child becomes the size of a building, and escapes from the laboratory, but not before losing a hand. Studying the hand, scientists and doctors find that the boy possesses surprising regenerative qualities. After doing some investigating, it is discovered that the boy is actually the result of Frankenstein's heart regenerating after being exposed to the radioactivity of the city's bombing in 1945. The boy is subsequently named Frankenstein.
Meanwhile, offshore, an oil rig is struck by what is believed to be an earthquake, but one of the surviving workers sees something monstrous moving underground. Later on, people and animals start disappearing all over the Japanese countryside, and a small village is soon attacked. Frankenstein is initially blamed for these disasters, and the JSDF begins hunting Frankenstein throughout the forests of Japan. Meanwhile, when the scientists go looking for Frankenstein to try and prove his innocence, they are attacked by the subterrarian monster Baragon, a dinosaur that has survived underground. The JSDF stop attacking Frankenstein after the true culprit is revealed, and Frankenstein soon engages Baragon in battle, eventually killing the monster after the forest has been set on fire. In the international version, Frankenstein then attacks a giant octopus after defeating Baragon, but he is overpowered and dragged into the ocean, his fight with Baragon having weakened him. In the Japanese and American versions, Frankenstein falls victim to an earthquake when the ground collapses beneath him and he is swallowed by the Earth.
This film contains examples of the following:
- 20 Minutes into the Past: Made in 1965, but seems to be set circa 1961 given the scene after the prologue is set in 1960 and not a whole lot of time seems to pass.
- Breath Weapon: Baragon can shoot fire out of his mouth.
- Chairman of the Brawl: Dr. Bowen hits Frankenstein with a chair when he starts acting out of control, and seemingly threatens Sueko.
- David vs. Goliath: Frankenstein vs. Baragon, then at the end, Frankenstein vs. Oodako.
- Digital Destruction: The Media Blasters reconstruction of AIP's cut is a half-digested, poor representation of the theatrical cut. All of the exclusive footage is missing (because Toho doesn't seem to have them on hand in 'scope for some bizarre reason), the location titles are badly recreated, several lines are missing, and the AIP logo at the end is absent among other oddities. This is mostly because Media Blasters had to work off of the atrocious '80s UPA video master of the film, however. The earthquake ending was also only available in LD quality for some reason.
- Everything's Better with Dinosaurs: Baragon is a dinosaur whose species has somehow survived into modern times, much like Godzilla and other kaiju.
- From a Single Cell: In conjunction with the nuclear example below, Frankenstein grew from a heart into a whole person after being exposed to radioactivity, and this ability would later be passed onto his successors, Sanda and Gaira.
- Gentle Giant: Frankenstein counts after he's grown to the size of a house.
- Giant Equals Invincible: Averted. Baragon is never successfully attacked by the military, so no one really knows how impervious he may or may not be. Frankenstein, however, is vulnerable to tank fire, but he posesses a regenerative healing ability.
- Gratuitous Laboratory Flasks: Used to eerie effect in the credits as the camera follows some chemicals flowing through different tubes and flasks. Dr. Lisendorf's lab in the World War II Cold Open also has a pretty impressive array of chemistry equipment... all of which he smashes after Frankenstein's heart is confiscated.
- Helping Hands: The scientists manage to lose Frankenstein's severed hand when it crawls away on its own.
- Herr Doktor: Nazi scientist Dr. Lisendorf, especially in the postwar scenes.
- I Love Nuclear Power: It can cause hearts to grow into mutant children, for one.
- Mad Scientist: Dr. Lisendorf is a delightfully maniacal example, with a Mad Scientist Laboratory in a German castle full of Gratuitous Laboratory Flasks filled with Technicolor Science.
- Motherly Scientist: Sueko is this to the ever-growing Frankenstein.
- Neck Snap: Frankenstein finishes his fight with Baragon by snapping its neck.
- Never Trust a Title: Frankenstein never conquers the world, nor does he even try to.
- Pet the Dog: Sueko occasionally does this to Frankenstein, which is heart-warming at best.
- Recycled Soundtrack: Varan vs. the Patrol Boat Uranami, from Varan, the Unbelievable, was reused and modified here as the piece that plays when Frankenstein's heart is being transported from Germany to Japan. Additionally, Varan vs. the Fighter Bomber Neptune was reused as the Tank Corps' march.
- Tragic Monster: Frankenstein certainly counts, although he seems more like a John Steinbeck character than anything.
- What the Hell, Hero?: At one point the two (male) scientists taking care of Frankenstein decide to test his regeneration by slicing one of his arms off!