Four years after the events of The Wolf Man, grave robbers break into the Talbot family crypt in Wales. Unfortunately, they pick the night of a full moon and the first thing they do is remove the wolfsbane in the casket. The body of Larry Talbot (Lon Chaney Jr.) is bathed in moonlight. Later that night, the Cardiff Police find him with the head wound that was believed to have killed him, and he is treated by Doctor Frank Mannering (Patrick Knowles) of Queens Hospital.
During the next night, Talbot transforms into the Wolf Man and kills a police constable. The next day, he remembers everything and begs Doctor Mannering to call the police before he kills again. Inspector Owen (Dennis Hoey) has reports of Larry’s death and believes this is an imposter. He becomes violent and is tied to his bed. The Doctor and Inspector Owen travel to the village of Llanwelly and find Larry’s body missing and a very dead grave robber. Larry escapes from the hospital by biting through his restraints. He goes in search of the Gypsy sorceress Maleva (Maria Ouspenskaya), who suggests he might find help with Doctor Frankenstein. When they arrive in Vasaria, they learn that Frankenstein is dead and his estate a ruin. Larry hopes to find the Doctor’s notes in the ruin somewhere.
He transforms and kills a young girl and a mob of villagers chase him down. He falls through the weakened flooring and, returned to human form, finds Frankenstein’s monster (Bela Lugosi) trapped in ice. He frees the creature, hoping he can find the notes, but he cannot. So Larry poses as a buyer for the estate to meet Baroness Elsa Frankenstein (Ilona Massey), Frankenstein’s daughter. Doctor Mannering arrives, having tracked the Wolf Man across Europe. He advises Larry to commit himself to a mental hospital, but Larry doesn’t think that will hold him. Frankenstein’s monster crashes a village festival and Larry helps it escape from the villagers.
Elsa and Mannering agree to help the villagers. They meet with Larry and the monster at the ruins, Elsa finds the notes, and Mannering studies them. Larry asks that his life energy be transferred to the monster, but Mannering plans to drain both of them of life. Overcome with curiosity, though, he revives the monster to full power. This happens on a full moon and both monsters emerge with full strength. The monster carries Elsa away, as the Wolf Man attacks him. The innkeeper, Vazec (Rex Evans) believes they are all in cahoots together and blows up the dam overlooking Castle Frankenstein. Instead of fire this time, the castle is destroyed by water. Elsa and Mannering escape, but the monster and the Wolf Man, still fighting, are swept away.
The film was directed by Roy William Neill, the script written by Curt Siodmak as a sequel to both The Ghost of Frankenstein and The Wolf Man. In the beginning, Lon Chaney was to play both characters, but it would take too great a toll on the man. Bela Lugosi was cast as the monster, but test audiences laughed every time he opened his mouth, so they cut out all his dialogue. But one of the details lost in that process was that the monster had been blinded in a previous movie, so there was no explanation of why he stumbled around with his hands out in front of him and it became the way the monster walked from then on, not only in parodies but in real films. It was released to lukewarm reviews, but it began a series of so-called Monster Rallies starring at least two characters. The Wolf Man had grossed nearly a million dollars and producer George Waggner proposed this meeting of monsters.
The fact that Frankenstein takes place in the past and the Wolf Man in the present was ignored by everyone. Dwight Frye, who had been in virtually every Frankenstein film, died shortly after its release. Bela Lugosi collapsed from exhaustion caused by his 35 pounds of makeup. During World War II, the village of Frankenstein was no longer in Germany, but in the imaginary country of Vasaria. The dog Bruno in the film was Lon Chaney’s German Shepherd, who had played a wolf in The Wolf Man. Clips from the film appear in many other movies, including one that is ignored in the background by Robert De Niro and Uma Thurman having sex in Mad Dog and Glory, and in the background of Alien vs. Predator. The movie was panned by critics, but it set the stage for monster vs. monster movies, not only with the Universal stable of creatures, but pairings like King Kong vs. Godzilla, and Freddy vs. Jason. It also saved Universal Studios from bankruptcy.