Mr. X is Straub's original and startling take on the theme of the doppelganger. Ned Dunstan's birthday is fast approaching, and every year on this date, Ned experiences a paralyzing seizure in which he is forced to witness scenes of ruthless slaughter perpetrated by a mysterious and malevolent figure in black who Ned calls Mr. X.. "Ned has been drawn back to his hometown, Edgerton, Illinois, by a premonition that his mother, Star, is dying. Before she loosens her hold on life, she imparts to Ned the name of his father, never before disclosed, and warns him that he is in grave danger. He discovers that he is shadowed by an identical twin brother who can pass through doors and otherwise defy the laws of nature; he becomes the lead suspect in three violent deaths; he investigates the secret shadow world within Edgerton; he learns to "eat time" and remembers the one occasion when he and his sinister brother united into a single being. Finally, at the moment of battle, he must call upon everything he has learned to save his own life.