Welcome to Book 2! To sum up what didn't happen in Book 1: Tristram is not born, Toby's character is not sketched, and his hobby-horse is not acquired.
Tristram begins by explaining that he needs an entirely new book in order to tell Toby's story, and he immediately digresses into a paragraph about King William's wars.
Toby has the hardest time telling the story of the siege (almost as hard a time as Tristram has telling his own story …), because he doesn't exactly know how the battle happened. He thinks that if he just had a big map, he could figure it out.