After staking out, obtaining, and then being forced to give up her first boyfriend, Josh, all Cammie Morgan wants is a peaceful semester. But that's easier said than done when you're a CIA legacy and go to the premier school in the world ... for spies.
Despite Cammie's best intentions to be a normal student, danger seems to follow her. She and her friends learn that their school is going to play host to some mysterious guests; code name: Blackthorne. Then she's blamed for a security breach that leaves the school's top secret status at risk.
Soon Cammie and her friends are crawling through walls and surveilling the school to learn the truth about Blackthorne and clear Cammie's name. Even though they have confidence in their spying skills, this time the target is tougher (and hotter), and the stakes for Cammie's heart-and her beloved school-are higher than ever before.
Warning, this summary contains major plot spoilers
The story begins as Cammie and her mother are on their way to Washington, D.C. where Cammie is 'debriefed' on her relationship with "the subject" (AKA Josh). Cammie returns to school at the start of a new semester, upon which her mother's requests that she 'just be herself'.
Soon, Cammie, Bex, Liz, and Macey, discover a mysterious 'Blackthorne', which turns out to be a top secret school for boy spies! On a CoveOps mission, the sophomore class of the Gallagher Academy visits Washington DC. Their mission is to reach their teacher at the Smithsonian's display of Dorthy's ruby slippers without having a tail. When Cammie and Bex believe they have no tails, they decide to split up just in case. Then a boy comes up to Cammie, and appears to flirt with her . Cammie thinks that he is just a normal boy, and tells him where she is going and that she is going to meet her teacher. However, when she reaches her teacher, she realizes that the boy is actually a spy that was on her tail. As the boy says "Hi again, Gallagher Girl" and her teacher Mr. Solomon says "Nice job, Zach", Cammie realises that "Zach" is from the Blackthorne Institute for Boys and impresses both Zach and Mr. Solomon by displaying this knowledge, saying "Hi, Blackthorne Boy" in reply.
The story continues to follow Cammie as she navigates through her semester of school, which becomes MUCH harder when fifteen students from Blackthorne come to live at the Gallagher Academy until the end f the school year. Josh pursues Cammie, constantly hanging around her. During a CoveOps mission in Roseville, she meets Josh and another friend, DeeDee, and learns that the two are dating. She freezes, not knowing what to do, until Zach comes over and saves her. He holds her hand, and DeeDee is mislead to think that the two are together. She invites Cammie and Zach to attend a dance in town. Unfortunately, the entire sophomore class overhears Cammie's embarrassing moment via the comms unit. However, on the night of the dance, the Gallagher Academy has planned an exam for its students in the form of a ball. Zach asks Cammie to dance after dancing with everyone else, but when Cammie's strapless bra starts slipping, she leaves Zach so that she can change. But much to her frustration, none of the teachers will let her back to her dormitory! She reaches an empty corridor, but then Zach finds her there. He says that he knew she was coming there, because he had seen her come to the corridor when she was upset. Just then, a Code Black sounds, and Cammie grabs Zach and tries to reach safety. When people suspect Cammie of triggering the Code Black, Zach denies being with her when it happened. This makes Cammie feel a mix of emotions and her friends become suspicious of Zach, and they decide to launch a mission to investigate the visiting Blackthorne students.
Cammie, Bex, Liz, and Macey plant bugs in the east wing where the Blackthorne students are staying, and start to get close to them to get information. Cammie and Zach go on a study date, in which Zach reveals that he found the bugs in the rooms. He drops them on the table in front of Cammie, and whispers, "I'm not all bad, Gallagher Girl." He starts to leave the library and looks back. "Then again, I'm not all good, either!" In a CoveOps lesson, when testing voice-analyzer rings that can sense lies, Zach asks Cammie if she had fun on their study date, and takes her excuse of not answering as a "yes". Later Cammie and Zach are on a date in town. Zach leads her down an alley-way and says "Just so you know, Gallagher Girl, I'm going to kiss you now." (This is the part when every girl reading starts to fangirl) But DeeDee interrupts them before anything can happen. (This is the part when every girl reading starts to resent DeeDee) When they walk down the street behind DeeDee and Josh, Cammie notices a possible tail. When Zach denies even seeing the man before, Cammie feels her voice-analyzer ring vibrate, telling her that this is a lie. Cammie ditches Zach in town and runs all the way back to the Gallagher Academy. The school is completely empty. She catches Mr. Mosckowitz as he is about to pass out, saying that someone's stolen the alumni list and it's outside the walls of the school. Cammie finds the other Gallagher Girls and go looking for the alumni list that they assumed had been stolen by the boys. However, the school is in code black mode- a process that locks all exits) so they must use a secret passageway. They trace the boys via tracking device to an abandoned warehouse that the Academy owns. Cammie is alone on the roof of a building when she bumps into Zach and cinches his arm.He rolls and pins Cammie's hands. They learn that the boys had nothing to do with the disk being stolen, and that it is actually the boy's teacher that has stolen the disk. However, when they get the disk back, they learn that everything was just a test to see whether or not the girls were able to trust and work together with the boys that they will have to work with someday. At the end, the boys are leaving the Academy so they can take finals back at their own school. Cammie says goodbye to Zach, and he dips and kisses Cammie in the school's foyer. (All fangirls love Zach now) Zach says "I always finish what I start". When Cammie says to Zach "so this is goodbye?" and then he winks and says, "Come on, Gallagher Girl, what are the odds of that?".