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Olga Ustinov, Sigmund Baginov's sister, asks for his help: Francine, Grazia and Mike, three friends with whom they had shared an adventure in a cave in the tourist village "The Jungle" fifteen years earlier, on which occasion Sigmund began to stutter, have disappeared. Returning to "The Jungle", Sigmund and Olga, with the help of Nathan and Legs, set out in search of the missing, led by the speleologist Karen. When they find Francine, the woman shows signs of a terrible mutation... While Sigmund descends into the underground caves to search for Olga, the military, who have discovered the Martian origin of the mutant beings, intend to recover their spores and destroy their nest. Thus an expedition begins, led by Colonel Rotkopf, which will cross the path of Nathan, Legs, May and Karen, who descend into the caves to find Sigmund and Olga, as well as Mike, now transformed and having escaped from the infirmary. The bottom of the caves seems to exert an inexplicable attraction on Martian beings...