Reversal is a 35-chapter fan-fiction novel by the pseudonymous jespah.
The story is told from the perspective of Ensign Lili O'Day, the sous-chef of the Enterprise, and how her late 2157 dreams are synchronized with events in the mirror universe, as she meets and falls in love with the counterpart to the deceased Major Jay Hayes, Lieutenant Commander Doug Hayes.
By approaching the Lafa System, Lili's nascent psionic abilities begin to come out, and she unwittingly begins to make contact with Doug, which begins as purely sexual contact. As they continue making contact, the two fall in love as the Calafan governments in both universes seek to use Lili and the Jennifer Crossman (mirror) as pawns to ensure public support for bringing in new High Priestesses. In both universes, a Calafan named Polloria is behind the ruse.
In gratitude for exposing the plot, the Calafans find a way to drop the veil between the two universes and send Doug to the prime universe. But all is not right, as Lili is injured. Dr. Phlox has to work to save her as Doug worries and waits to learn his fate.