Groton was a mute, dimwitted homicidal maniac who lived in Venice Beach, California. He was the assistant of a scientist named Doctor Durae who, unknown to Groton, was in fact the last male heir to the legacy of Frankenstein. He lived in Durae's laboratory, which was situated behind a secret panel at Doctor Durae's Creature Emporium on the boardwalk. Durae was attempting to develop a serum that would cure various physical and mental abnormalities through the study of blood. He used an early version of the serum to temporarily keep Groton’s homicidal rages in check. But Durae also required Groton's savage quirks, and would often deny him the serum whenever he needed to rely upon the brute's bloodlust.
In March of 1969, Durae sent Groton to acquire another test subject for his experiments. He would often target his victims based upon the people who would visit his Creature Emporium. Groton came upon a woman named Joanie Fontaine and decapitated her with an ax. He brought both her head and body back to Durae who reattached them and found a way to resurrect her. Afterward, Durae gave Groton another shot of his formula to keep his rage in check.
Later, Durae sent Groton out again to find more victims. He went out to the beaches at Rocky Point where he attacked two lovers named Bob and Laura. After hacking them to pieces, he left Bob’s remains on the beach and brought Laura back to the laboratory.
A few nights later, Durae sent him out again to gather more test subjects. Groton came upon a gang of bikers antagonizing a hippie girl named Samantha with the intention of raping her. He hacked away at them with his ax and brought Samantha back to the laboratory.
Before long, Judith Fontaine, the sister of Groton's first victim, Joanie Fontaine, accompanied her lover Mike Howard to Durae's lab where they learned the truth about the doctor's experiments. After substantial pontificating, Durae ordered Groton to kill them. Judith managed to escape from the lab and Groton gave chase. He pursued her across the rooftop, but the police arrived on the scene and a detective named Sgt. Martin shot Groton to death.
- Joanie Fontaine - Decapitated by Groton under the pier
- Laura - Decapitated by Groton at Rocky Point
- Bob - Dismembered by Groton at Rocky Point
- Rico - Dismembered by Groton under the pier
- Biker #1 - Dismembered by Groton under the pier
- Biker #2 - Dismembered by Groton under the pier
- Samantha - Death unconfirmed; possibly killed by Groton
- Woman in Durae's lab - Unknown; Likely killed by Groton
- This was the last film role by actor Lon Chaney, Jr. Aside from some random grunts and snarls, Chaney has no speaking lines in this film. This was due to Chaney suffering from lung and throat cancer. Originally, the character of Groton was to have speaking lines conveyed through a narrative, but Chaney's voice was so raspy that Al Adamson and Sam Sherman decided not to use the voiceover work.
- Despite the title of the film, Chaney's character has no screen time with either Dracula or Frankenstein's Monster.
- The character of Groton appears to be a pastiche of the classic Doctor Jekyll and Mister Hyde archetype. At times, he comes across as a jovial, simple-minded old man caressing a puppy, but whenever Durae requires it, he transforms into a mutated, homicidal maniac.
- There are a number of inconsistencies with the portrayal of this character. In one scene, Doctor Durae uses his formula to stimulate Groton's metabolic change, while in other scenes it appears as if he is attempting to alleviate his symptoms.
- Trailers for the film billed Chaney's character as the "Mad Zombie", which would explain some of his physical characteristics throughout the film, but at no point does the movie ever make reference to the notion that he is a zombie.
- Early in the film, Doctor Durae (J. Carrol Naish) refers to Chaney's character as Grodin. Throughout the rest of the film though, he calls him Groton.