Part one
Victor Frankenstein is a man training as a doctor, engaged to Elizabeth Fanshawe. After Victor's younger brother, William, drowns, Victor renounces his belief in God and declares that he would join forces with the Devil if he could learn how to restore his brother to life.
Told as a flashback to the early 21st century with a wraparound sequence narrated by the orangutan Lawgiver (John Huston) in "North America - 2670 A.D.", this sequel follows the ape leader, Caesar (Roddy McDowall), years after a global nuclear war has destroyed civilization. In this post-nuclear society, Caesar tries to cultivate peace between the apes and the surviving humans. A gorilla general named Aldo (Claude Akins), however, opposes this and plots Caesar's downfall. Caesar is married to Lisa (Natalie Trundy), the female ape of the previous film, and they have a son, named Cornelius (Bobby Porter) in honor of Caesar's father.
The story takes place in Istanbul, where a violent criminal organization led by Spider-Man ("Spider's Gang") surfaces in the city with counterfeit United States dollars. A small task-force consisting of Captain America, Santo and Captain America's girlfriend Julia arrives to help local police stop Spider-Man and his gang.
Big and burly African-American soldier Eddie Turner (Joe De Sue) stepped on a land mine while serving in Vietnam and lost both arms and legs. His physicist fiancee Doctor Winifred Walker (Ivory Stone) thinks she's found help for him in her white former teacher and colleague Doctor Stein (John Hart) who has recently won a Nobel Peace Prize for "solving the DNA genetic code".
Baron von Frankenstein neglects his duties towards his wife/sister Katrin, as he is obsessed with creating a perfect Serbian race to obey his commands, beginning by assembling a perfect male and female from parts of corpses. The doctor's sublimation of his sexual urges by his powerful urge for domination is shown when he utilizes the surgical wounds of his female creation to satisfy his lust. He is dissatisfied with the inadequate reproductive urges of his current male creation, and seeks a head donor with a greater libido; he also repeatedly exhibits an intense interest that the creature's "nasum" (nose) have a correctly Serbian shape.
The opening titles set the film in "North America – 1991." Armando (Ricardo Montalbán) explains that in 1983 (ten years after the end of Escape from the Planet of the Apes, which was set two years ahead of its theatrical release date), a disease killed the world's cats and dogs, leaving humans with no pets. To replace them, humans began keeping apes as household pets. Realizing the apes' capacity to learn and adapt, humans train them to perform household tasks. By 1991, American culture is based on ape slave labor (just as Cornelius described would happen in the previous film). It is also suggested that the North America of the 1990s is at least partly a police state, as apes and humans are being watched at all times.
After Baron Henry von Frankenstein creates a bride his monster, he decides to make arrangements for a lavish wedding at the Transylvania Astoria Hotel. Many monsters are invited to the wedding including Count Dracula and his son, Ron Chanley the Werewolf, the Mummy, Dr. Jekyll and Mr. Hyde, the Invisible Man and his family, and the Creature from the Black Lagoon. Once the wedding guests arrive, they terrify guests and staff. Frankenstein's assistant Igor is jealous of the monster and wants the bride for his own. So Igor plots to steal her which doesn't go according to plan when the bride ends up snatched up by a Pterodactyl and lands in the clutches of a giant gorilla named Modzoola.
By day, Dr. Frankenstein (Gordon Mitchell) works innocuously in his lab. But at night, he works to perfect Mosaico (Xiro Papas), a monstrosity pieced together from dead bodies. Once completed, the behemoth escapes from the lab and embarks on a killing spree. Local beauties begin popping up dead, murdered in a variety of gruesome ways, as authorities attempt to stop Mosaico's rampage.
A mad scientist (J. Carrol Naish) descended from
the original Dr. Frankenstein takes to murdering young women for experimentation in hopes of reviving his ancestor's creation, with help from his mute assistant Groton (Lon Chaney, Jr.). Dracula (played by Roger Engel under the pseudonym "Zandor Vorkov") comes to the scientist promising to revive Frankenstein's monster in return for a serum which will grant him immortality.
The films opens with a trio of grave robbers (led by a man named Lynch) delivering a corpse to Baron Frankenstein (Cotten) and his assistant Dr. Marshall (Müller), for obvious reanimation purposes.
Victor Frankenstein, a cold, arrogant and womanizing genius, is angry when his father forbids him to continue his anatomy experiments. He ruthlessly murders his father by sabotaging the old man's shotgun, consequently inheriting the title of Baron von Frankenstein and the family fortune. He uses the money to enter medical school in Vienna, but is forced to return home when he impregnates the daughter of the Dean.
Following the events of Planet of the Apes, time-displaced astronaut Taylor (Charlton Heston) and the mute Nova (Linda Harrison) are riding on horseback through the desert of the Forbidden Zone. Without warning, fire shoots up from the ground and deep chasms open. Confused by the strange phenomenon, Taylor investigates a cliff wall and disappears before Nova's eyes.
Aliens, running a traveling circus as a cover, revive a vampire, a werewolf, a mummy and Frankenstein's monster with a plan to use them to take over the world. They want to discover the reason that these monsters are so frightening to Earthlings. They then plan to create an army of such monsters using their findings.
Baron Victor Frankenstein is staying at a boarding house while a former fellow-scientist resides in a nearby insane asylum, slowly dying through lack of oxygen in the brain.