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Dark Mirror (1993)
by Diane Duane
Amazon.com Product Description (ISBN 0671794388, Paperback)One hundred years ago, four crewmembers of the "U.S.S EnterpriseTM crossed the dimensional barrier and found 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 which had 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 is to destroy from within, to replace a Federation Starships with one of their own. Their victims are the crew of the "U.S.S. Enterprise NCC-1701-D, who now find themselves engaged in combat against the most savage enemies they have ever encountered, themselves.
(retrieved from Amazon Fri, 04 Jan 2013 02:02:57 -0500)
One hundred years after an accident with the dimensional barrier introduced four shipmates to an alternate universe, the sinister members of that universe discover a doorway into Picard's time, and plot to replace his Enterprise with theirs.