Godzilla: The Series

Leviathan (1x8)

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Nick and H.E.A.T. go deep beneath the sea in order to investigate the mysterious disappearance of noted scientist Dr. Alexander Preloran and his own research team, and there H.E.A.T. discovers a hidden spacecraft belonging to a race of advanced, hostile non-humanoid aliens that hope to conquer the Earth with the help of a pet giant monster.