Doctor Who: A Different Body A Different Destiny
What if rather than Tremas the Master had absorbed the body of Nyssa on Traken? Now biologically female the Master decides to set a different type of trap for the Doctor. Going by the name Samantha "Sam" Ret she tricks the Doctor into becoming one of his companions. However, when romantic feelings start to develop between her and the Fifth Doctor things become complicated.
Characters
Fourth Doctor, Fifth Doctor, Adric, Master, exact
https://www.fanfiction.net/s/1300587...ferent-Destiny
If so inclined, enjoy.