Mysterious dark creatures known only as "Phantoms" have wiped out most of humanity. Four survivors, who might well be the last humans left, have found each other and are sharing a quiet, peaceful existence. The group consists of Masashi, a kind and dependable teenage boy; Sanae, a shy teenage girl; Yaeko, a bolder teenage girl; and a little girl named Anna. They feel as if they are living in a perpetual "after school" state, waiting for the rest of the human race to come home. As they spend their days together, they begin to notice strange occurrences and realize that the monsters have not yet left Earth. Will they discover what really happened to their planet?