Sam Wheat was murdered during a date with his girlfriend Molly. Sam's spirit, in a mysterious manner, remains on the Earth, unseen to normal people. After following his murderer, Sam realizes that his death was not an accident, and that molly is in danger. Sam's ghost try to warn Molly, but she does not know it is really there. Then he meets a spiritualist named Oda Mae Brown, who can hear the dead. Sam speaks to hear and convince her to help him save Molly.
Sometime in the future, the polar ice caps have melted and the world's oceans and seas have covered the land. People are few and far between, living in small communities at sea or sailing from one to another as traders. All the people seek dry land.. something nobody has seen.
After the world is decimated by a man-made plague, a battle of literally Biblical proportions ensues between the survivors.