As the World Shatters

Set after Timelash but before the Trial of a Time Lord, Peri has been traveling with the Doctor for some time. However, when a perfectly normal day takes a catastrophic turn for the worst, the Doctor is forced to make a massive sacrifice in order to save his companions life. Suddenly alone and trapped in a universe almost the same but also very different from her own, Peri finds herself face to face with someone she never expected to see again, living a very different life, with a very different Doctor. This may have been the life she had wanted once, such a long time ago, but now she missed her own life.... her own Doctor. Can she find a way to turn things back to how they were? or will she have to learn to live with the universe the way it has now become.
This story is told through Peri's own eyes and memories as both the story of her new life and how it came about are gradually revealed, culminating in a dramatic conclusion and a choice Peri never expected to be forced to make.