On a snowy night, a mysterious stranger, his face swathed in bandages and his eyes obscured by dark goggles, takes a room at The Lion's Head Inn in the English village of Iping in Sussex. The man demands that he be left alone. Later, the innkeeper, Mr. Hall (Forrester Harvey) is sent by his wife (Una O'Connor) to evict the stranger after he makes a huge mess in his room while doing research and falls behind on his rent. Angered, the stranger throws Mr. Hall down the stairs. Confronted by a policeman and some local villagers, he removes his bandages and goggles, revealing that he is invisible. Laughing maniacally, he takes off his clothes, making himself completely undetectable, and drives off his tormenters before fleeing into the countryside.The stranger is Dr. Jack Griffin (Claude Rains), a chemist who has discovered the secret of invisibility while conducting a series of tests involving an obscure drug called monocane. Flora Cranley (Gloria Stuart), Griffin's fiancee and the daughter of Griffin's employer, Dr. Cranley (Henry Travers), becomes distraught over Griffin's long absence. Cranley and his other assistant, Dr. Kemp (William Harrigan), search Griffin's empty laboratory, finding only a single note in a cupboard. Cranley becomes concerned when he reads it. On a list of chemicals is monocane, which Cranley knows is extremely dangerous; an injection of it drove a dog mad in Germany. Griffin, it seems, is unaware of this, having learned about monocane in English books printed before the incident.On the evening of his escape from the inn, Griffin turns up at Kemp's home. He forces Kemp to become his visible partner in a plot to dominate the world through a reign of terror, commencing with "a few murders here and there". They drive back to the inn to retrieve his notebooks on the invisibility process. Sneaking inside, Griffin finds a police inquiry underway, conducted by an official who believes it is all a hoax. After securing his books, he attacks and kills the officer.Back home, Kemp calls first Cranley, asking for help, and then the police. Flora persuades her father to let her come along. In her presence, Griffin becomes more placid and calls her "darling." When he realizes Kemp has betrayed him, his first reaction is to get Flora away from danger. After promising Kemp that at 10 o'clock the next night he will murder him, Griffin escapes and goes on a killing spree. He causes the derailment of a train, resulting in a hundred deaths, and throws two volunteer searchers off a cliff. The police offer a reward for anyone who can think of a way to catch the Invisible Man.The chief detective (Dudley Digges) in charge of the search uses Kemp as bait, feeling Griffin will try to fulfill his promise, and devises various clever traps. At Kemp's insistence, the police disguise him in a police uniform and let him drive his car away from his house. Griffin, however, is hiding in the back seat of the car. He overpowers Kemp and ties him up in the front seat. Griffin then sends the car down a steep hill and over a cliff, where it explodes on impact.Griffin seeks shelter from a snowstorm in a barn. A farmer hears snoring and sees the hay, in which Griffin is sleeping, moving. The man notifies the police. The police surround the building and set fire to the barn. When Griffin comes out, the chief detective sees his footprints in the snow and opens fire, mortally wounding him. Griffin is taken to the hospital where, on his deathbed, he admits to Flora that he had tampered with something that was meant to be left alone. After he dies, his body gradually becomes visible again.
Output: Question: What drug seems to offer the secret to invisibility?, Answer: Monocane