Frankenstein begins with Edward Van Sloan stepping from behind a curtain and delivering a brief caution before the opening credits:
How do you do? Mr. Carl Laemmle feels it would be a little unkind to present this picture without just a word of friendly warning: We are about to unfold the story of Frankenstein, a man of science who sought to create a man after his own image without reckoning upon God. It is one of the strangest tales ever told. It deals with the two great mysteries of creation; life and death. I think it will thrill you. It may shock you. It might even horrify you. So, if any of you feel that you do not care to subject your nerves to such a strain, now's your chance to uh, well,––we warned you!!
In a village of the Bavarian Alps, a young scientist, named Henry Frankenstein, and his assistant Fritz, a hunchback, piece together a human body, the parts of which have been collected from various sources, including stolen freshly buried bodies in a cemetery, and the bodies of recently hanged criminals. Frankenstein desires to create human life through electrical devices which he has perfected. He sends Fritz to a school where Dr. Waldman, Henry's old medical professor, teaches, to steal a brain; Fritz drops the normal brain and has to take the brain of a criminal.
Elizabeth, his fiancée, is worried over his peculiar actions. She cannot understand why he secludes himself in an abandoned watch tower, which he has equipped as a laboratory, and refuses to see anyone. She and a friend, Victor Moritz, go to Dr. Waldman, and ask Waldman's help in reclaiming the young scientist from his experiments. Waldman tells them that Frankenstein has been working on creating life. Elizabeth, intent on rescuing Frankenstein, arrives just as Henry is making his final tests. He tells them to watch, claiming to have discovered the ray that brought life into the world. They watch Frankenstein and the hunchback as they raise the dead creature on an operating table, high into the room, toward an opening at the top of the laboratory. Then a terrific crash of thunder booms, and Frankenstein's electric machines flash, crackle and buzz. Shortly, the hand of Frankenstein's creature begins to move. This prompts Frankenstein to shout 'It's alive!'.
The manufactured creature, despite its grotesque form, initially appears to be a simple, innocent creation. Frankenstein welcomes it into his laboratory and asks his creation to sit, which it does. He then opens up the roof, causing the creature to reach out towards the sunlight. Fritz enters with a flaming torch, which frightens the creature. Its fright is mistaken by Frankenstein and Waldman as an attempt to attack them, and it is chained in the dungeon. Thinking that it is not fit for society and will wreak havoc at any chance, they leave the creature locked up, where Fritz antagonizes it with a torch. As Henry and Waldman consider the creature's fate, they hear a shriek from the dungeon. Frankenstein and Waldman run down and find that the creature has strangled Fritz. The creature lunges at the two but they escape, locking the creature inside. Realizing that the creature must be destroyed, Henry prepares an injection of a powerful drug and the two conspire to release the creature and inject it as it attacks. When the door is unlocked the creature lunges at Frankenstein as Waldman injects the drug into the creature's back. The creature falls to the floor unconscious.
Henry collapses from exhaustion, and Elizabeth and Henry's father arrive and take him home. Henry is worried about the creature but Waldman reassures him that he will destroy it. Later, Henry is at home, recovered and preparing for his wedding while Waldman examines the creature. As he is preparing to vivisect it, the creature awakens and strangles him. It escapes from the tower and wanders through the landscape. It has a short encounter with a farmer's young daughter, Maria. She is not afraid of him and asks him to play a game with her in which they toss flowers into a lake and watch them float. The creature enjoys the game, but when they run out of flowers he thinks Maria will float as well, so he throws her into the lake where, to his puzzlement, she drowns. Upset by this outcome, the creature runs away.
With preparations for the wedding completed, Henry is serenely happy with Elizabeth. They are to marry as soon as Waldman arrives. However, Victor rushes in, saying that Doctor Waldman has been found strangled. Henry suspects the creature. Meanwhile, the creature enters Elizabeth's room, causing her to scream. When the searchers arrive, they find Elizabeth unconscious on the bed. The creature has escaped.
Maria's father arrives, carrying his daughter's body. He says she was murdered, and the villagers form a search party to capture the creature, and bring it to justice (dead or alive). In order to search the whole country for the creature, they split into three groups: Ludwig leads the first group into the woods, Henry leads the second group into the mountains, and the Bürgermaster leads the third group by the lake. During the search, Henry becomes separated from the group and is discovered by the creature, who attacks him. The creature knocks Henry unconscious and carries him off to an old mill. The peasants hear his cries and they regroup to follow. They find the creature has climbed to the top, dragging Henry with him. The creature hurls the scientist to the ground. His fall is broken by the vanes of the windmill, saving his life. Some of the villagers hurry him to his home while the rest of the mob set the windmill ablaze, killing the entrapped creature inside.
At Castle Frankenstein, Frankenstein's father, Baron Frankenstein, celebrates the wedding of his recovered son with a toast to a future grandchild.