It wasn't a bird -- but it couldn't be a man! Lt.William Kenlon had watched the incredible creature circling the submarine Sea Serpent in the darkness of the night, and he could barely believe what he saw. The giant winged monster was human -- and it was intent on some purpose that involved the sub and its crew. Then the creature landed -- and suddenly, impossibly, the Sea Serpent was in another world. A world of the far future; where the land was uninhabitable and humanity as Kenlon knew it had died out. A world in which the strange bird-men of the air warred with the even stranger denizens of the sea for domination. And in that bizarre battle for survival, the men from the 20th century were the vital factor!