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On the day that he is to hang for the murder of his brother, Geoffrey Radcliffe is visited by his old friend, Dr. Frank Griffin, who secretly administers a serum that renders the condemned man invisible. Unseen, Geoffrey slips out of prison to find the real murderer of his brother. Soon after, Frank is visited by Inspector Sampson of Scotland Yard, who questions him about the formula for invisibility that his brother discovered nine years earlier. Knowing that the serum caused insanity in his brother, Frank struggles to discover an antidote before his friend goes mad. Meanwhile, Geoffrey, attempting to elude the police, takes refuge in Frank's office, where he learns that Willie Spears, a lowly miner in the Radcliffe mines, has been promoted to the position of mine superintendent by Geoffrey's cousin and heir, Richard Cobb. Suspicious, Geoffrey follows Spears and terrorizes him into confessing that he saw Cobb kill Geoffrey's brother. Geoffrey then confronts Cobb with his guilt, but Cobb escapes and runs into Inspector Sampson, who orders the house surrounded, but Geoffrey disguises himself as a police officer and escapes. As Geoffrey descends into madness, his fiancée, Helen Manson, and Frank try to restrain him, but he tricks them and escapes to continue his pursuit of Cobb. Geoffrey chases Cobb to the mine, and as the two struggle on the coal elevator, Geoffrey is shot by the police and Cobb falls to his death as the cart overturns. Before he dies, however, Cobb confesses to the murder. Geoffrey, suffering from his bullet wound and pneumonia, returns to Helen and Frank. To save his life, Frank administers a blood transfusion that turns out to be the antidote that restores his sanity and visibility.