When the Old Guard (including Andy for ~reasons) arrive at the Prospect of Whitby after those 100 years, Booker isn't there to meet them. Now they have to track him through the years and find out why.

(The reason is up to the author. Maybe he lost his immortality and had five extremely confused kids, maybe he's been captured, maybe he was in the middle of rescuing kittens from a burning orphanage and completely lost track of time)