Can never remember what the heck a second or third cousin is … always going back to Wikipedia for a refresher. It’s confusing enough. In this case, the third cousins are the children of second cousins, who are the children of cousins.
Now that we’ve got that straight, what is the story? Who the hell knows? It’s just a title and a cover!
One idea puts us in a small town (sort of like Lyttleton, a harbor town on a hill, scenic enough for a tourist trade with the occasional cruise liner docking there for day trips into a nearby interior city) where some number of people are all looking for a certain person, for initially unknown reason(s). The problem is, everyone and every business in the town has recently relocated. Like Christchurch, there was some event that caused this upheaval, and now no one is quite sure where anyone or anything is anymore. Likewise, buildings disappear over the weekend (a la Christchurch). Whether this is all due to a natural disaster, like the earthquake here, or for some other (possibly unknown) reasons, is yet to be decided. It may just be the way things are in this place!