Doutless there are times in everyone's life when they wish they could have a second chance.  For Commander William T. Riker of the U.S.S. Eneterprise that second chance became a bizarre reality.  Back in 2361 Riker, (then a lieutenant aboard the U.S.S. Potempkin) led an evacuation of the science team on Nervala IV, a planet that was developing a powerful distortion field that would soon prevent all access to the surface.  The evacuation was successful and Riker, the last to beam out, barely made it back to the ship.  Neither he nor the crew of the Potempkin ever suspected that a freak transporter accident had left a duplicate of Riker on the surface, stranded and alone.

Now, eight years later, during one of the brief windows of time during which access to Nervala's surface is possible, Commander Riker beams down with an away team to retrieve the computer database that had been left behind by the science team.  He suddenly finds himself face-to-face with himself -- a self who has been marooned alone for eight years.

Thomas Riker, as he eventually named himself, is identical to William Riker up to the moment of the transporter accident.  Gut the last eight years has made all the difference in the world between the two.  While Will confidently continued his Starfleet career, earned promotions and became First Offecer of the Enterprise, Thomas is still a lieutenant, socially disoriented after so many years alone, and still in love with Deanna Troi.  When he arrives aboard the Enterprise, he is devastated to learn that she no longer has romantic feelings for him.

However, Thomas is not entirely surprised to discover that Will's breakup with Deanna was Will's choice.  Highly ambitious, Will's career came before romance.  Thomas assures Deanna that he's not the same, that he won't ever leave her, and he pursues her until she slowly caves in.  All the love Deanna once felt for Will returns, and Thomas has hopes that after so long, they will finally be together.

Thomas' relationship with Will is not so hopeful.  Will is unnerved by his twin's existence, and during their mission to retrieve the computer database from Nervala IV, Will hounds his subordinate, demanding a level of performance that Thomas cannot give.  Only after Thomas nearly falls to his death does Will truly realize that his twin is no threat to him.  They can both live their lives, even pursue Starfleet careers.  But both know ho much those careers mean to them.  Deanna knows it, too.  When Captain Jean-Luc Picard arranges a post for Thomas aboard the U.S.S. Ghandi, Deanna says goodbye to Thomas, knowing that their romance is over, as it was with Will eight years before.