A practicing psychiatrist from the planet Pharlem escapes to Earth when his planet is under siege by a hostile population of creatures who dwell deep underground and can only surface at night. Once here, he discovers that he has changed in two disturbing ways: His eyes are terribly sensitive to light, and he cannot tolerate any food or drink...only blood. He does not wish to kill anyone but runs into some difficulty controlling this compulsion.

He meets a psychologist, Dr. Leah Mendelssohn, who is in urgent need of a psychiatrist to replace her colleague who left. His true background unbeknownst to her, she invites "Dr. Theodore Pharlem" to sit in on one of her group sessions, accepting his assurance that he will supply copies of his credentials in due time. "Ted" manages to control his blood lust just enough to avoid killing two thugs who try to attack him--but they are changed in unexpected ways.

The doctor meets Rita, who reminds him of his wife back home in Pharlem, and falls dangerously in love with her. When the body of a missing young woman is found in a wooded graveyard, covered with a cloak of unidentifiable material, the Department of Homeland Security gets involved, thinking this is the work of a sophisticated terrorist organization. The chase is on.