An American photo-journalist accompanies two Japanese scientists in a bathysphere to study the Cromwell Current deep in the mid-Pacific Ocean. Suddenly, a volcano erupts at close hand. Shock waves from the vent tear the diving bell loose from its research ship. The three men are saved by a high-tech submarine, the Alpha. Unbelievably, its captain, Craig McKenzie, is more than 200 years old and the sub was launched in 1805. With the stranded research crew aboard, it heads for its home port in the underwater Utopian city, Latitude Zero. A rival 200-year-old scientist, Dr. Malic, sends his Black Shark sub after them. When he fails to destroy it before it reaches Latitude Zero's protective shell, he designs a plan to lure McKenzie out again. He captures Dr. Okada, a Japanese scientist bound for their Latitude Zero, to bait his trap. McKenzie must penetrate Malic's base on Blood Rock to free him and his daughter. Malic has a host of human/ animal hybrids waiting to greet the intruders.