Dark Mirror

Stardate 44010.2: the twenty-fourth century. Humanity's greatest dreams have become reality. Along with dozens of other sentient races, the people of Earth have formed the United Federation of Planets - a galactic civilization that governs much of the known universe for the good of all. Over the past two centuries, mankind has tamed its basest instincts, and reached the stars...
But suppose it hadn't happened that way at all? Suppose instead humanity's darkest impulses, its most savage, animalistic desires had triumphed? Suppose that the empire mankind made out in the stars was one ruled by terror, where only those willing to brutalize their own kind and their neighbors could survive?
One hundred years ago, four crewmembers of the U.S.S. Enterprise crossed the dimensional barrier and found just such an empire. A mirror image of their own universe, populated by nightmare duplicates of their shipmates. Barely able to escape with their lives, they returned thankful that the accident that brought them there could not be duplicated. Or so they thought.
But now the scientists of that empire have found a doorway into our universe. Their plan: to destroy from within, to replace one of our starships with one of theirs. Their victims: the crew of the U.S.S. Enterprise NCC 1701-D.