Guardian is a continue on from Inbetween- following Amelia's transition into Guardian.
Amelia thought that when she healed mentally from her death, that'd be it. No more flashbacks of Dylan. No more Dylan of any kind. She'd be able to heal herself and get onto teasing Apollo about something that everyone except him picked up on. Learn how to become a Guardian and do her job properly, while she waits for the 'birth' of her daughter and re-builds her own World again. After all, she's got eternity, right?
Something happens that may disrupt the balance of both the Living and the Inbetween... and suddenly, eternity isn't a promise for any of them. It's a threat.