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In Tirol in the late 19th century, when twins are born to the ruling Baron de Berghman, the youngest, Anton, is sent away due to a family legend that the youngest twin will kill the oldest. Two decades later, the elder twin, Gregor (Boris Karloff) arranges for Anton's (also Karloff) return so that the brothers may rule their country jointly. Upon Anton's arrival, mysterious disappearances and murders occur -- and when the local peasants accuse Gregor, he assumes his more popular twin's identity.

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A group of college students travel south to Florida on spring break with their friend Karen, whose father, Dr Richardson, is a marine biologist currently studying jellyfish. At the party, Karen's friends insult and ridicule Dr Richardson's assistant, Egon, who is in love with Karen. Egon creates a murderous jellyfish mutant which begins a killing spree and eventually brings Karen to his secret laboratory in the swamp.

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