Jace, along with Alec and Isabelle, slays a demon in the Pandemonium Club while Clary Fray watches.
The next day, he confronts Clary in an alley because he thinks she has more powers than she lets on. She runs away after a distressing call from her mother, and Jace follows her, finding out she's been attacked by a poisonous Ravener demon. He carries her to the Institute, where he lives.
When Clary recovers, she decides to go back and check out her apartment. Fearing it might be dangerous, Jace accompanies her.
Good thing, too, because they get attacked by a Forsaken, which gives Jace the opportunity to show off his mad fighting skills.
Afterward, Jace has to follow Clary around for a bit while she does some investigative work, trying to figure out where her mother is. Along the way, they discover Clary has a weird magical memory block.
With Alec and Isabelle, they try to get the High Warlock of Brooklyn, Magnus Bane, to remove the block. He can't. Then they have to go save Simon from a pack of vampires. Long story.
After he's healed, Jace takes Clary on a midnight picnic. Smooching ensues.
Clary realizes where the Mortal Cup is hidden, and Jace prepares to retrieve it.
Unfortunately, Hodge steals the Cup and turns it and Jace over to Valentine.
While being held captive by Valentine, Jace learns that Valentine is his real father.
Clary arrives, and convinces Jace that even though Valentine is his dad, he's still evil.
Jace almost follows Valentine through the Portal to Idris, his homeland, but he decides to stay with Clary instead.
At the end of the book, he takes Clary on a fun flight through the city on a stolen flying motorcycle.