Dr. Frankenstein (Colin Clive) believes himself free of his creation (Boris Karloff) but the Monster survived the inferno by falling into an underground cistern.
The Monster skulks about the countryside for a while and eventually meets Dr. F's mentor, freaky old Dr. Praetorius (Ernest Thesiger).
Dr. P uses the Monster to get Dr. F to take his experiments in a new direction. Pervy Dr. P's idea is to create a mate for the Monster, then presumably observe what happens. Dr. F reluctantly agrees (his fiancee is held hostage) but, upon bringing the pretty hot Bride (Elsa Lanchester) to life, she totally rejects the hideous Monster.
Depressed, the Monster orders Dr. F to leave, then pulls a switch blowing the tower, Dr. P, the Bride and himself to atoms.