Sir Tristan goes back so far no one knows where he came into folklore. In Arthurian Legend Sir Tristan is a member of the Round Table. For a long time he was one of the most famous romantic characters of all time. On one quest he is sent to fetch the bride Iseult for the King only to have the two of them inadvertently drink a love potion that puts them helplessly in love. The have many trials that test their love. The first known appearance of Sir Tristan in Literature is in Celtic Folklore
In the year 3000, Earth is under siege by alien invaders, and the battle does not go well for humanity. In this darkest hour, the legendary King Arthur is accidentally awakened by the young Tom Prentice. Arthur wastes no time in summoning Merlin and collecting Excalibur to combat the alien threat, but is still lacking his Knights of the Round Table. To reassemble the knights, Merlin casts a spell which reincarnates their spirits into individuals around the world. Unfortunately for Sir Tristan, he is brought back as the young woman Amber March literally moments before she is to be married. "She" leaves the groom at the altar, and goes off to serve Arthur in his struggle.
As an interesting side note, the Camelot 3000 miniseries was written by Mike Barr, who would later go on to write the Ultraverse comic Mantra which centers around the story of a man reincarnated as a woman. Barr cited Tristan as one of the main inspirations for Mantra, and the two characters seem to have much in common personality-wise. (They also seem to have similar bad luck when it comes to weddings).
Tristan is tremendously unhappy with her new gender and spends much of her time attempting to prove her worth to Arthur and trying to find some way to restore her lost manhood. Her situation is further complicated with the reincarnation of Isolde, her lover from her previous life who wishes to resume their relationship regardless of Tristan's new gender. To confuse matters further, Tom begins to develop feelings for "Amber," feelings she does not reciprocate.
Tristan's strong desire to regain her manhood brings her into contact with Morgan Le Fay, who promises to restore Tristan's true gender, but only if she betrays Arthur. Furthermore, Morgan has brought Amber's fiance into her service, transforming him into a monster who still remembers his love/hate for the woman who left him.