The Entropy Effect by Vonda N. McIntyre

The U.S.S. Enterprise is summoned to transport a dangerous criminal from Starbase prison to a rehabilitation center, the brilliant physicist Dr. Georges Mordreaux, accused of promising to send people back in time, and then killing them instead.

But there's more at stake than just a few lives. For Mordreaux's experiments have somehow thrown the entire universe into a deadly time warp. All of existence is closing in on itself, and only Spock can stop the Entropy Effect.

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