Main articles: Fenric and Cartmel Masterplan
See also: Dragonfire (Doctor Who), Silver Nemesis, and The Curse of Fenric
Fenric is a loose story arc covering Ace's time in the TARDIS with the Doctor from the end of Season 24 to near the end of Season 26. It involved a malevolent entity known as Fenric, that was beaten by the Doctor in a game of chess some unknown time in the past and banished to a different dimension. Since then, he had been setting up a series of events leading to his return. This was also part of the initial story setup for the Cartmel Masterplan which never came to full realization.
Read more about this topic: Story Arcs In Doctor Who, Classic Series