It introduces two giant, hairy humanoids called Gargantuas, which spawned from the discarded cells of Frankenstein's monster from the previous film and are described as brothers. The Green Gargantua is violent and savage, preying upon human beings; as he lives in sea water, he is given the name Gaira(ガイラ, from kai, "sea"). The Brown Gargantua had been raised in captivity, and is docile and gentle; because he resides in the Japan Alps, he is called Sanda(サンダ, from san, "mountain"). The film follows the investigation and military engagements of these creatures until their climactic confrontation in Tokyo.