The world is endangered when a vortex opens in the ocean, and Mark must find where its going and some how stop it before all the water is drained away.

While investigating an underwater volcano, Mark is stricken with sudden pain in the shoulder. This is not the first time he's felt pain for no reason, and his instincts lead him to a deserted beach and, inexplicably, to a frontier town of the old west called Land's End. There he meets his identical twin, Billy, another water-breather who washed up on a beach with no knowledge of his true identity. Billy takes Mark's place aboard the Cetacean, leaving the Man from Atlantis to face the villains of Land's End.