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.

When a sea quake opens a fissure and Mark investigates, he finds himself in the world of "Romeo and Juliet".