Scientist Arnold Kramer's theory that an enemy power is digging a complex of tunnels beneath U.S. soil is verified by findings of the Los Alamos Atomic Detection Center. The Pentagon determines that the Communist Chinese are building the subterranean network and orders Comdr. Jonathan Shaw to help Kramer avert disaster. Their assignment is to prevent the Chinese from detonating the stockpiled nuclear weapons that they have delivered into the tunnels by a single supply shaft under the Pacific Ocean. After Shaw has set up his staff, including geologist Tila Yung, inside an extinct Hawaiian volcano, he prepares to attack the Chinese supply tunnel, while Kramer builds a borer equipped with a laser beam head. Before the mission can be accomplished, however, Shaw, Kramer, Tila Yung, and the others are captured by General Chan Lu. Kramer heroically gives his life to save the others, who escape. Shaw then activates the mechanism on one of the nuclear weapons, and a tremendous blast erupts inside the tunnel, destroying the Chinese forces and creating an enormous mushroom cloud over the Pacific.