Are you ready to be so scared your brain leaps right out of your skull and absconds with your socks? Then prepare for Frankenstein. (The film actually opens with actor Edward Van Sloan warning you that you're about to be really, really scared. Spoiler: The film isn't actually all that scary. Movie-goers were easier to scare back there in 1931.)
Anyway; scary stuff starts. Henry Frankenstein is startled to discover he's called "Henry" because he's Victor in the novel. But he recovers, and does his Frankenstein thing, which is stitching together a body out of corpse parts with the help of his assistant, Fritz.
To complete his work, Frankenstein needs a brain. Fritz tries to get him a perfect healthy brain, but instead grabs hold of an evil criminal brain. Bad move, Fritz.
Meanwhile, Frankenstein's fiancée Elizabeth is worried. Why is Henry locked away in his abandoned watchtower when he should be out getting hitched? Elizabeth and her friend Victor Moritz go to question Frankenstein's old medical professor Dr. Waldman. Dr. Waldman explains that Frankenstein's trying to create life. Why not create life the usual way, with some good ol' fashioned baby-making? Apparently that's not good enough for Henry.
Elizabeth, Victor, and Waldman go to confront Frankenstein just as he's finishing his last tests to create life. Using a thunderstorm for power, Frankenstein animates his monster while they all watch. Triumph!
Well, there are complications.
The monster doesn't speak; it just makes noises. Frankenstein keeps the monster in darkness. When he first brings it out into the light, the monster seems pleased…but then Fritz threatens it with a torch. The monster gets scared. Frankenstein and Waldman think the monster is attacking, and chain it in a dungeon because they're kind of awful.
While they try to figure out what to do with the scary monster they've made, Fritz threatens the monster again, and the monster kills him. Frankenstein and Waldman decide they have to destroy the monster. They inject it with a drug, knocking it out.
Frankenstein decides he's had enough of monsters, and goes back to Elizabeth. Meanwhile Waldman prepares to dissect the creature. But he didn't kill it first. So when he starts to cut into him, he wakes up and strangles Waldman. (You can hardly blame him.)
The monster leaves the lab and goes out into the country. He meets a small child, Maria, and the two play together. As part of playing, the monster throws Maria in a lake, and she drowns. Someone needs to teach this monster how to behave on playdates.
Meanwhile, Henry and Elizabeth are getting ready to be married. They're just waiting for Waldman, who keeps not coming, because—oops—they find out he's been strangled to death. Frankenstein figures it's the monster. Then there's a scream in the house, prompting much scrambling and rushing about. The monster ends up terrorizing Elizabeth, who faints. (Women were always fainting in movies in 1931.)
Maria's body is found, and the villagers organize to go kill the monster. Henry leads one group, and ends up confronting the monster alone. The monster knocks Frankenstein unconscious. Again, you can hardly blame him; this is all Frankenstein's fault after all.
The monster takes Frankenstein to a windmill. Frankenstein wakes up, the two struggle, and the monster throws Frankenstein from the mill. Frankenstein isn't killed (because test audiences really didn't like it when he was killed in an early version of the film). The villagers light the mill on fire, killing the monster…or at least appearing to kill him; he ends up coming back for Bride of Frankenstein.
Frankenstein's taken back home, where he recovers and marries Elizabeth. His father, Baron Frankenstein, raises a toast to a future son. You'd think after the horrible job he did raising his monster, Henry would be reluctant to have any kids, but maybe he figures he learned something—like, don't make your child out of dead body parts.