Chelsea and Andre are the first demigod children of Chaos to ever be conceived; they were born into a relative peace-time, and as such, their problems don't quite stem from monsters hunting them. It's more bills, business management, and messy normal people stuff. Chelsea's just out of college and works at Hermes Express; impressing him with her delivery skills enough for him to call her his herald (nothin' more than a title btw). Andre, meanwhile, starts a fledgling prankster-for-hire company with Leo and the Stolls (who are all a year Andre's senior), and starts a relationship with Rachel. Everyone else is off doing their own thing pre-TOA, but Leo gets back earlier, while Jason and Piper still break up. Notably, the hidden world of mythologies in America is entering an age of exploration - after Percy and Annabeth's encounter with the Kanes, the gods are becoming aware that the previously well-maintained barriers between their pantheons are cracking.
The other five (Chase, Rowan, Tammy/Tommy, Ava and Aiden) are all Kronos Army-remnants. Andre and Chelsea are dispatched to hunt them down and bring them back to camp, but Chase unleashes a Cychreides that wreaks havoc in the Diamond District of Manhattan. As Andre and Chelsea battle it, it serves as the quintet's distraction to escape to Westchester, and eventually Boston.