In the prologue, we time warp back to 16th-century France, where we see a dark and mysterious boy with crazy back scars force an entitled and arrogant boy to swear his loyalty. Crazy-back-scar boy tells self-entitled boy, Chauncey, that he is half mortal, half fallen angel.
Back in the present day, Nora's bio class is on the human reproduction unit, and the bio teacher switches up the seating chart, placing Nora next to Patch. Some creepy stuff, like the feeling of an otherworldly presence hovering around, has been going on with Nora since her father's death, but when she starts interacting with Patch, things get weirder. For example, after a night out at the movies and library with Vee, some lunatic in a ski mask attacks Vee's car while Nora is driving it home. Funny thing, though—when Nora brings the car back to Vee's to survey the damage, there's nothing but a small crack in the windshield. Color Nora confused.
Luckily a new distraction arises when Nora and Vee meet Jules and Elliot, two prep school dreamboats. Elliot is set to transfer to Coldwater High, the school Nora and Vee attend, and he turns up in Nora's gym class where he promptly flirts with her and asks her out on a date. Nora's game, but Patch continues to lurk on the fringe, piquing her interest and showing up quite frequently when he's not expected.
Vee, Nora, Elliot, and Jules go on a double date to Delphic amusement park, where—surprise—Patch shows up (again). Nora ditches the others and goes on a rollercoaster ride with Patch, where another freaky thing happens: She has a really vivid experience of flying out of the rollercoaster car, but then the ride ends safely. Is her imagination going cuckoo on her, or is something else up? By the time the ride is over, Nora can't find the rest of her group anywhere, so she gets a ride home with Patch. Consummate gentleman that he is (sarcasm alert), Patch walks Nora to her door, invites himself in, and makes her tacos. Cue the kissing.
The next day, Nora goes shopping with Vee. She notices the ski-masked stalker again, so she and Vee make a plan to catch the stalker by having Vee wear Nora's jacket to entice the stalker to following her, and then Nora will follow them both to crack the mystery.
Major plan fail, though: Vee is attacked and lands in the hospital. Back at school, Nora has to see the new school psychologist, a young blond named Miss Greene, for weekly appointments she's had to attend since her father's death. She does some research for an eZine review, and she stumbles on an article about how Elliot was questioned in eighteen-year-old Kjirsten Halverson's suspicious hanging death, which was eventually ruled a suicide. Needless to say, she's a little jumpy when Elliot enters the computer lab, and she refuses his invitation to go out again.
Nora and Vee go out to the Borderline to celebrate her release from the hospital and to sleuth around for info about Patch since he works there. Vee also invites Jules and Elliot and reveals that she's been dating Jules, though he's a no show that night (stomach bug, according to Elliot). Vee and Nora have differing opinions about the ski mask wearer—Nora thinks it's Elliot; Vee thinks it's Patch.
Nora's mom makes a rare appearance back at home, but she's quickly called back to the office, leaving Nora home alone to catch a glimpse of the ski mask fleeing the scene after ransacking her room. Nora calls the police, but when they arrive, everything seems to be back in order. So odd.
A few days later, the police show up again to ask about an attack on Marcie Millar right after she and Nora got into a name-calling match at the library. They float the idea that maybe her bio-partner boyfriend (their words—Nora isn't as fast and loose with the term "boyfriend" for Patch) had something to do with it. She asks Patch about it on their date that night. He says he knows nothing about it.
Also on their date, Nora gets a glimpse of two long scars down Patch's back. She begins to put two and two together and does some research on fallen angels. Google tells her they're an evil brood who tempt humans to devilish deeds by communicating directly to their minds and possessing human bodies during the month of Cheshvan. She also reads about Nephilim, the wicked (wicked bad, not wicked cool) offspring of fallen angels and humans. As if she's not freaked out enough, Elliot shows up the next morning to ask her to go camping during spring break. When she refuses, he threatens her.
Nora goes to Portland—the nearest big city to Coldwater and close enough to drive to for a day of shopping or an evening out—to do some sleuthing about Kjirsten. Turns out, Vee is in Portland, too, and she texts Nora to join her at a party with Jules and Elliot. On her way to meet Vee, Nora exchanges her coat and hat for directions from a homeless woman. As Nora heads off, she hears shots and finds the homeless woman dead. Say what?
She goes to call police, but the body vanishes, so she calls Patch to come get her. Vee texts that she's headed back to Coldwater with Jules and Elliot.
On the drive home, Patch has car trouble, and he and Nora have to seek shelter in a seedy motel. In a fondling sesh, Nora touches Patch's scar and is sucked back in time, where she sees Patch's ex-girlfriend Dabria trying to talk Patch out of his plan to become human. She tells him he can become a guardian angel instead if he saves a human life. He asks for the name of someone about to die, and she gives him Nora's, adding that she's in danger of death because someone wants to kill her. That someone is Patch. (Cue thunder claps and scary music.)
To get Nora to trust him, Patch lets her touch the scars again. She finds out that his plan to become human involves killing his Nephil vassal. Patch is surprised that Nora recognizes Dabria as Miss Greene and to learn that she is still on earth. Patch takes Nora home so he can go investigate what Dabria is doing, but Dabria shows up at Nora's and tries to kill her in a jealous rage. She also reveals that Nora is related to Chauncey and that Patch planned to use her as a sacrifice to help destroy Chauncey.
Nora manages to hide from Dabria as she sets the house on fire. Patch arrives in time to get Nora out, and then he goes searching for Dabria while Nora goes to look for Vee at the movies. Vee calls Nora and tells her that she, Elliot, and Jules broke into the school to play hide-and-seek. Elliot says Nora better come play, too, or he will kill Vee. Um… whoa.
Patch and Nora drive to the school. Patch goes in first and is gone so long that Nora goes in on her own. It's time for the big reveal. The dude in the ski mask is Jules (most of the time… Dabria was the ski-masked villain behind Vee's attack), and Jules is really Chauncey. He has injured Elliot and Vee and intends to kill Nora to torture Patch for possessing his body all these years.
He corners Nora on a rafter high above the gym floor, but rather than have him shoot her, Nora flings herself to the ground. If she has to die, she decides she'd rather sacrifice herself so that Patch can get his human body instead of having Jules kill her.
It would be pretty grim to see our protagonist die after all these pages with her, so imagine our relief when we find out that Patch rejects her sacrifice and her soul returns to her body. The act has still killed Jules, though, and Patch becomes Nora's guardian angel.