After the prologue, the story kicks off after the last time we see Ten and Rose in the alternate universe together at The End of Time Part 2. It follows the Doctor and Rose's lives in this parallel world from Rose's perspective.
Rose and the Doctor end up in an alternate universe. The stories of Merlin and Arthur and the Knights of the round table are true. The Doctor and Rose are separated, but not for long. But there's something dark that is closer than they could ever know.