In his mountain-top lab in the abandoned watchtower, viewed from outside against a stormy night sky, brilliant medical student Dr. Henry Frankenstein has assembled a human body, stitched together from parts of different corpses stolen from graves. The young scientist, newly engaged but away from his intended bride, is madly obsessed with his equipment and experiments while violent claps of thunder sound from the storm raging outdoors. The incomplete, lifeless creation is covered and stretched out in his laboratory on an operating table. He has constructed machinery that will take the electrical power from a lightning storm and input the electricity into the lifeless body through electrodes in the neck.

In his primitive-looking laboratory, a whole network of electrical wires, globes, rings of steel, banks of gauges, dials, phallic-shaped levers and switches will harness the zapping power of lightning bolts and bring the creature to life. The howling, violent storm outside makes him expectant:

This storm will be magnificent. All the electrical secrets of Heaven. And this time we're ready, eh Fritz? Ready.

From under the blanket covering the corpse, a blackened, scarred arm protrudes outward. Henry calms Fritz's jittery nerves as he caresses the arm of the inanimate corpse:

There's nothing to fear. Look. No blood, no decay. Just a few stitches. And look, here's the final touch. (Frankenstein uncovers a bandaged and wrapped head.) The brain you stole, Fritz. (He shows Fritz that he has installed the brain that was stolen from the medical school.) Think of it. The brain of a dead man waiting to live again in a body I made with my own hands, with my own hands. Let's have one final test. Throw the switches.

[It is most unusual that Frankenstein didn't notice the obvious characteristics of the criminal brain installed in the body.]

Crackling sparks fly when switches are thrown during the final test. Frankenstein anticipates that fifteen minutes before the storm's height will be the perfect time to electrify the Monster's body. They are interrupted by incessant knocking heard at the heavy watchtower door, and an irritated Frankenstein sends Fritz down to answer it: "Whoever it is, don't let them in...Of all times for anybody to come!" A harried-looking Fritz descends the long, dark stone staircase, muttering to himself about being bothered "at this time of night - got too much to do." The bent-over servant refuses entrance to Dr. Waldman, Elizabeth, and Victor, speaking to them through the door's window: "You can't see him. Go away." Then, the hunchback servant hobbles back up the stone stairs, awkwardly pausing to pull up his sock.

Frankenstein lets them in when Elizabeth begs for shelter from outside. A distraught Henry climbs back down with Fritz, not wishing to be disturbed as he protests: "You must leave me alone." He explains that he is all right, although he looks pale, haggard, and his eyes flash with anger. The mad, passionate scientist is worried about their intrusion (and particularly concerned about his fiancee's presence) during his final experiment:

Henry: Elizabeth, please, won't you go away? Won't you trust me, just for tonight?
Elizabeth: You're ill. What's the matter?
Henry: Nothing. I'm quite all right, truly I am. Oh, can't you see? I mustn't be disturbed. You'll ruin everything. My experiment is almost completed.
Elizabeth: Wait a moment. I understand. I believe in you. But I cannot leave you tonight.
Henry: You've got to leave!

Victor accuses him of being a madman: "Henry, you're inhuman. You're crazy." Frankenstein takes this as a challenge from a rival and brags:

Crazy, am I? We'll see whether I'm crazy or not. Come on up...

He leads them up to his ultra-secret laboratory, locks the door behind them (and pockets the key: "I'm forced to take unusual precautions"), and forcefully orders them to sit down. He boasts again about his experiment to Victor: "A moment ago you said I was crazy. Tomorrow we'll see about that." Fritz screams at Dr. Waldman who has wandered over to the corpse: "Don't touch that!" His yell is accentuated by more thunderclaps.

To his old teacher, Frankenstein (at first calmly) explains how he has made new discoveries beyond ultraviolet rays that were earlier taught to him by Waldman at the university. He boasts competitively about his experiments that have led to new revelations:

Frankenstein: Dr. Waldman. I learned a great deal from you at the University about the violet ray, the ultra-violet ray, which you said was the highest color in the spectrum. You were wrong. [Note: Infra-red is the highest color in the spectrum.] Here in this machinery I have gone beyond that. I have discovered the great ray that first brought life into the world.
Waldman: Oh! And your proof?
Frankenstein: Tonight, you shall have your proof. At first, I experimented only with dead animals, and then a human heart which I kept beating for three weeks. But now, I'm going to turn that ray on that body and endow it with life.
Waldman: (skeptically) And you really believe that you can bring life to the dead?
Frankenstein: That body is not dead. It has never lived. I created it. I made it with my own hands from the bodies I took from graves, from the gallows, anywhere! Go and see for yourself.

He permits Waldman to look at the body he has made with his own hands.

In a remarkable creation sequence in the watchtower during the raging storm, he has rigged an apparatus to take the inanimate, artificial body on a moveable platform to the open skylight at the rooftop of the tower where it can electrified by a lightning strike. The astonished witnesses and onlookers watch and hear the defiantly mad, zealously hysterical Dr. Henry Frankenstein theatrically convince his audience of his work:

Quite a good scene, isn't it? One man crazy - three very sane spectators.

With startled eyes, they witness the bizarre experiment in horror - an event not unlike the creative sexual act (with preparation, ascending movement, penetration into the opening, jolts of orgasm, shrieks of ecstasy, and the birth of a new being). Dr. Frankenstein and Fritz roll down a blanket (from head to foot) covering a white-shrouded, lifeless corpse. Then, they roll back the white sheet (from foot to head), revealing a monstrous cadaver underneath. Amidst the crackling of transformers, dynamos, and electrodes, with Fritz's help, the table is moved to the rooftop, where it is repeatedly struck by lightning, to harness the awesome energy of the storm. Lightning flashes, sparks, and electric arcs jump from machine to machine, jolting life into the inanimate monster strapped to the operating table. Finally, the immense platform-table is then lowered back down into the laboratory.

After its descent, at first there is no sign of movement or life - nothing seems to have happened and the creature fails to respond. But then, the creature's bare right hand that is hanging free twitches, in close-up - a promising sign that the Monster is coming to life. Dr. Frankenstein hysterically shouts like an overjoyed child about his recreation of life. He calls his creature an 'it' rather than a 'he', foreshadowing his future attitude toward the Monster:

Look! It's moving. It's alive. It's alive....It's alive, it's moving, it's alive, it's alive, it's alive, it's alive, it's alive! Oh - in the name of God. Now I know what it ["feels like to be God" - this phrase at the end of the sentence was censored and removed] ...

A mad and uncontrollable Henry is restrained by Waldman and Victor as his cries are drowned out by lightning bolts, howling winds and thunder.

The next day, in the company of Victor and Elizabeth, the mad doctor's father, a blustery aristocratic Baron Frankenstein (Frederick Kerr) suspects that something is wrong with his son, and they are covering up the truth:

You think I'm an idiot, don't you? But I'm not! Anyone can see with half an eye that there's something wrong. And I have two eyes, and pretty good ones at that. Well, what is it?...What's the matter with my son? What's he doing?...Why does he go messing around an old ruined windmill when he has a decent house, a bath, good food and drink, and a darn pretty girl to come back to? Ha, will you tell me that?

On the eve of their forthcoming marriage, the Baron suspects that Frankenstein is distracted by "another woman," rather than believing that he is passionately carrying on with his solitary experiments: "I understand perfectly well. There's another woman - and you're afraid to tell me. Pretty sort of experiments these must be!" When the arrival of the town's burgomaster Herr Vogel (Lionel Belmore) is announced by a maid, the Baron snorts at the pompous authority figure:

Baron: Well, tell him to go away.
Maid: But he says it's important.
Baron: Nothing the burgomaster can say can be of the slightest importance.

The town's burgomaster is ushered in, offers flowers to Elizabeth, and asks about the impending wedding:

Burgomaster: What I really want to know is, when will the wedding be, if you please?
Baron: Unless Henry comes to his senses, there'll be no wedding at all.
Burgomaster: But Herr Baron, the village is already prepared.
Baron: Well, tell them to unprepare.
Burgomaster: Oh, but such a lovely bride! And such a fine young man, the very image of his father.
Baron: Heaven forbid!
Burgomaster: But sir, everything is ready!

Because everyone in the village, the Bride, and the Baron himself are "kept waiting," the Baron threatens to go visit his son, find the other woman ("I'm going to find her!"), and bring Henry home.

Since the last scene, Dr. Waldman has remained at the tower with Frankenstein in his laboratory. Frankenstein is relaxed and smoking a cigar. Waldman, however, is worried and tries to reason with him: "This creature of yours should be kept under guard. Mark my words. He will prove dangerous." Henry sincerely responds that danger is an accepted part of the risk of experimentation:

Henry: Dangerous! Poor old Waldman. Have you never wanted to do anything that was dangerous? Where should we be if nobody tried to find out what lies beyond? Have you never wanted to look beyond the clouds and the stars or to know what causes the trees to bud and what changes the darkness into light? But if you talk like that, people call you crazy. But if I could discover just one of these things, what eternity is, for example, I wouldn't care if they did think I was crazy!
Waldman: You're young, my friend. Your success has intoxicated you. Wake up and look facts in the face! Here we have a fiend whose brain...
Henry: The brain must be given time to develop. It's a perfectly good brain, doctor. You ought to know. It came from your own laboratory.

Dr. Waldman shocks him by explaining that he has mistakenly implanted a criminal brain in his creature:

The brain that was stolen from my laboratory was a criminal brain.

Frankenstein is disturbed by this bit of information, but rationalizes away the problem: "Oh well, after all, it's only a piece of dead tissue." Waldman predicts with an admonishing warning of the danger: "Only Evil can come of it! Your health will be ruined if you persist in this madness." Frankenstein affirms his sanity: "I'm astonishingly sane, doctor." Waldman thinks the worst: "You have created a Monster and he will destroy you." Frankenstein wants to experiment further, realizing that the Monster will develop as time passes: "Patience, patience. I believe in this Monster, as you call it. And if you don't, well, you must leave me alone." He still trusts that Elizabeth believes in him - but his father "never believes in anyone."

Up until this point, the brutish Monster "is only a few days old" and has been kept in total darkness, but will soon be revealed: "So far, he's been kept in complete darkness. Wait till I bring him into the light." At that instant, the plodding thud of shuffling footsteps are heard outside the room. "Here he comes," Dr. Frankenstein announces, as he proudly heralds the arrival of his 'son.' The lights are turned out. The giant monster has developed enough strength to shuffle forward (like a teetering toddler) and enter into the mad scientist's laboratory from the dark corridor.