Assistance

Hugh visits the dentist for a routine procedure when the examination turns up evidence of a mysterious double living inside Hugh's body. Hugh, completely freaked out, flees from the office and falls into the hands of a mysterious woman who offers to help him over his trauma. At first she seems kind, but as her intentions become clear, Hugh realizes she intends to delve into the mysterious doppelganger with ruthless efficiency and scarcely a thought for Hugh's welfare. (This short story originally appeared in Space and Time magazine.)