In which a remarkable day unfolds beneath the Atlantic as the Nautilus glides over the vanished plains of Atlantis and finds refuge within the volcanic heart of a submerged island, revealing a hidden world of strange geology, diverse fauna, and secret havens unknown to the surface world. The professor and his companions explore this vast natural citadel, marvel at its flora and fauna, and experience the rising tide enclosing them in this subterranean sanctuary before the Nautilus resumes its mysterious voyage.