Heaven isn't 100% perfect by design. The Underworld isn't 100% evil by accident. These Guardians travel between the world of Heaven and earth, and, when they fall, become dark angels in the Underworld. Built into the series is the notion of redemption, that every evil or struggle can be overcome through true love. True love heals in the gardens of the heart. The characters in this popular story struggle to find their true calling. Sometimes it's as an angel, sometimes as a dark angel, and sometimes as human.