Without warning, an alien spaceship attacks a Japanese moon base. Back on Earth, young Kenichi Ishikawa; his father, Dr. Yosuke Ishikawa; his friend Helen; and her father, Dr. Tom Wallace, witness the spaceship descending into the ocean. They go to investigate but are soon captured by a teleportation beam that brings them aboard the spaceship. Inside the spaceship, a human-looking woman appears to them and reveals that she is of a race called Zigra. By way of demonstrating Zigran technological prowess, she creates a gigantic earthquake that wreaks havoc in Japan. She previously caused two other earthquakes, one in Peru and the other in Arabia. She then tells her prisoners of Zigra's history and its great scientific advances—which, unfortunately, have resulted in the destruction of the planet; but in searching for a new home, Zigra has found Earth. The woman contacts authorities on Earth and orders them to surrender or she will kill her prisoners. Tom declares that the Zigran woman is insane, and, in anger, she sends the two men into a hypnotic trance. Kenny and Helen take action, successfully using the ship's control console to escape. Enraged, the Zigran overlord—a strange, shark-like being—orders the woman to go to Earth and kill the children. She says it would be simpler to kill all the people of Japan, but the great Zigra tells her that humans must be preserved so they can be used for food. Now, Gamera, intent on discovering the identity of the alien interloper, flies in to save the day and rescues the children and their fathers. The UN authorities, after questioning Kenichi and Helen, resolve to attack Zigra. The Defense Force jets scramble, but the Zigran spaceship makes short work of them with its powerful lasers. The alien woman, disguised as a normal human, arrives on earth and begins her search for Kenny and Helen. She hitches a ride with the Sea World dolphin trainer back to the facility, which the military is now using as its center of operations. She finds the two children, but before she can catch them, they call out for Gamera, who obediently appears.