After Jace cheats on her, just because of one small fight, Clary leaves the city she's grown up in. After less than twelve hours, when she's at the point of getting killed by a demon which was from a kind she wasn't aware off, she is saved by the Delights.
Fast, they become friends, family. She finds out about other things that have been going on in the Universe, apart from the wars she's been through. Things other than creatures on this world. Way darker secrets than she's used too.
The Delights' leader, Delight, or so they call her, is one big secret to them all. Always wearing a black cloak, desperate to not show any skin. They do not know anything about her.
But what they do know, is that the number of Sacra's is raising, and that they are the only ones that would be able to do something about it.
So they go to New York City, Clary's hometown, because there are the most demons to be found, and killed.