Robert Griffin (Jon Hall) kills two orderlies and escapes from the Capetown mental institution and sets out for revenge on the Herrick family. Sir Jasper Herrick (Lester Matthews), Lady Irene (Gale Sonder Grand), and their daughter Julie (Evelyn Ankers) are entertaining Julie’s new boyfriend, Mark Foster (Alan Curtis) in their home. Griffin enters and accuses the couple of leaving him to die in the African bush when he was injured on their safari.
The Herricks say they were told he was dead, but he demands a share of the diamond fields they discovered. Jasper says the diamond fields were lost through bad investments, but Robert threatens to sue them and they offer him a share in their Shortlands Estate. He says they should marry their daughter to him. Lady Irene drugs him and he passes out. They worry that he is completely mad, so they search his clothes and find the partnership agreement, which they steal, and then throw him out of the house. He nearly drowns but is saved by a Cockney cobbler named Herbert Higgins (Leon Errol).
Herbert decides to use the information he got from Robert to blackmail the Herricks, but Jasper calls on Chief Constable Sir Frederick Travers (Leyland Hodgson), who voids Roberts claims to the Herrick estate and orders him to leave town. On his way to London, he drops in on scientist Doctor Peter Drury (John Carradine) who wants someone on whom to test his invisibility formula. Robert volunteers to get his revenge. Invisible now, Robert forces Jasper to sign over his estate, and Jasper agrees to give his daughter’s hand to Robert, should he become visible again.
Robert sneaks into the scientist’s laboratory and observes how the doctor restores visibility to his dog with a blood transfusion. He breaks into the lab, knocks the doctor unconscious, and performs a transfusion on himself with the doctor’s blood. The doctor dies and Robert sets the lab on fire to cover his escape. He then changes his name to Martin Field and moves into the Herrick estate. Herbert tries to blackmail him and Robert gives him a thousand pounds if Herbert will kill the doctor’s dog. Robert starts losing his visibility one day with Julie and Mark present. He tricks Mark into coming down to the basement, knocks him out, and begins another transfusion with Mark’s blood. Constable Travers arrives and, with help from Herbert and Jasper, breaks into the cellar in time to save Mark’s life. Robert is attacked by the dog and killed.
The film was directed by Ford Beebe and written by Bertram Millhauser. It received mixed reviews, declared tired and old hat by some, but others thought it entertaining, particularly the special effects. John Carradine, at least, said he enjoyed working on it. This Robert Griffin is not related to the other Griffins in the previous films. The grand Shortland House of Lord and Lady Herrick also appeared in several Sherlock Holmes movies, as did a number of actors in this particular film. The dog, called Brutus by his master, was listed in the credits as Grey Shadow. Evelyn Ankers appeared in The Wolf Man, The Ghost of Frankenstein, Captive Wild Women, Son of Dracula, The Mad Ghoul, Jungle Woman, Weird Woman, The Frozen Ghost, The Pearl of Death, and Tarzan’s Magic Fountain. She was called Queen of the B’s, made 50 films between 1936 and 1950, then married actor Richard Denning and retired at 32.