Only the Good Spy Young

The fourth installment in the Gallagher Girls series follows Cameron "Cammie" Morgan, a junior at the Gallagher Academy for Exceptional Young Women, a prestigious boarding school that secretly trains young women as spies. Cammie has spent recent months under armed protection because the Circle of Cavan, an ancient terrorist organization composed almost entirely of double agents embedded within the world's intelligence services, has been targeting her for unknown reasons.

During winter break, Cammie stays in London with her best friend, Rebecca "Bex" Baxter, whose parents are top British intelligence agents. At the Tower of London ice rink, Zachary "Zach" Goode, a student from the Blackthorne Institute for Boys and Cammie's love interest, appears and tells her that their Covert Operations teacher, Joe Solomon, was supposed to meet him days ago but never showed. Zach vanishes before Cammie can press further.

Minutes later, the Tower's lights go out. In the chaos, Solomon himself appears, grabs Cammie, and pulls her onto Tower Bridge, insisting the Circle is coming for her. Bex's father arrives and orders Solomon to surrender, revealing the CIA wants him for questioning. Solomon grips Cammie's shoulders and urgently tells her to "follow the pigeons" when she returns to school. He makes her promise, then runs toward the rising edge of the drawbridge and leaps into the Thames.

Cammie is taken to a secret MI6 underground facility, where a tall, blue-eyed British agent interrogates her and declares that Solomon is the threat. Over the following days, the Baxters move Cammie and Bex through MI6 safe houses. The girls eavesdrop on a conversation involving Cammie's Aunt Abby, a CIA operative and her mother's sister, who reveals that a captured Circle operative confirmed Solomon was recruited by the Circle at age sixteen while attending Blackthorne.

Back at the Gallagher Academy, the school has been fortified with armed guards, sealed passageways, and bulletproof glass. Cammie's mother, Rachel Morgan, the headmistress, has been "temporarily detained." At the Welcome Back Dinner, Agent Edward Townsend, the blue-eyed man from the London interrogation, is introduced as the new CoveOps instructor. Cammie whispers the truth to her roommates: Solomon is Circle.

Her roommates process the revelation. Macey McHenry, a perceptive senator's daughter, argues Solomon's terror during a previous attack seemed genuine. Bex insists her parents would not lie. Elizabeth "Liz" Sutton, the group's resident genius, is torn. The four agree that Townsend likely knows more than he reveals and resolve to extract information from him. After weeks of fruitless surveillance, they inject an apple with Liz's experimental truth serum and swap it with his. The drugged Townsend confirms Solomon was Circle since sixteen and reveals the CIA has been detaining Rachel. Before passing out, he makes a devastating claim: Solomon killed Cammie's father, Matthew Morgan.

When Rachel finally returns to the school, Cammie asks directly whether Solomon killed her father. Rachel cannot deny it, saying only that amazing operatives are the best liars.

Remembering Solomon's cryptic instruction, Cammie finds her way to an old pigeon loft on the mansion's rampart, where she discovers a hidden room with chalkboards covered in Solomon's handwriting. A carrier pigeon bears an unsigned note asking her to meet at a location where they once practiced brush passes, a type of covert handoff made during a casual encounter. The girls sneak out, and Zach reveals he is working with Solomon. He tells Cammie that Solomon hid a journal in Sublevel Two, the school's secure underground archive, that "explains everything," and that Solomon himself activated the sublevels' defenses to keep the wrong people out.

Weeks later, Townsend takes the class on a field exercise to an amusement park, secretly using Cammie as bait. Solomon appears, gaunt and desperate. Cammie slaps him and screams that her father is dead because of him; Solomon does not deny it. CIA agents swarm the park and capture Solomon. The junior class watches their beloved teacher led away in restraints.

Rachel publicly confronts Townsend for endangering her daughter. Cammie resolves to break into Sublevel Two. After weeks of preparation, the four girls enter through a ventilation shaft, navigate laser grids, and trigger alarms that flood the space with lake water. They solve an ancient stained-glass puzzle and shout the Academy's traditional oath to satisfy a mechanical security system. Liz retrieves the journal just as Townsend breaches the wall, and they escape narrowly.

The journal, in Cammie's father's handwriting, documents Matthew Morgan's discovery that Solomon was secretly Circle, Solomon's confession, and their joint covert mission to destroy the organization from outside official channels. The final entries, in Solomon's hand, record Morgan's disappearance in Athens: He never arrived at a meeting, and after weeks of searching, the trail went cold.

Townsend reveals that a CIA transport was ambushed and Solomon was taken by the Circle. Zach sneaks into the school and explains that Solomon kept a second journal at Blackthorne documenting his recruitment. With Rachel's permission, the group travels to Blackthorne, where Zach leads the girls through whitewater rapids and over a waterfall to reach the school undetected.

Inside "the tombs," a network of caves within the mountains, Cammie discovers Blackthorne's original purpose: chains, steel tables, and instruments of torture reveal it trained assassins. Deep inside, they find Solomon's journal. But the Circle has followed: Solomon, beaten and bound, has been brought to this explosives-rigged chamber. Guards capture Zach and Cammie.

The woman leading the Circle operatives addresses Zach as "sweetheart" and asks him to introduce his girlfriend to his mother. Cammie is stunned: The operative who has been hunting her is Zach's mother, a former Gallagher Girl. A violent fight erupts. When a guard aims at Zach, his mother tackles the gunman, unwilling to let her own son die. Zach grabs the gun and fires at the explosives. Cammie runs through the tunnels as the mountain detonates behind her. She reaches a dead end behind a waterfall, where Zach's mother corners her and reveals they do not want to kill Cammie but want something inside her mind. She adds, "If you kill me, then who will take you to your father?" Before the Circle can seize her, Cammie leaps through the waterfall into the river below.

Cammie survives and is rescued. In the infirmary, Townsend shows her that Solomon is alive but unconscious, brought back by Zach. Townsend declares Solomon officially dead, ending the manhunt. Rachel arranges for Zach to finish the semester at the Academy and warns Cammie that the Circle will say anything to manipulate her; in her professional opinion, Matthew Morgan is probably not alive, though she cannot be certain.

Solomon remains in a coma. Zach, haunted and barely speaking, proposes that he and Cammie run away together to keep everyone safe. Cammie kisses him but refuses. In her final CoveOps report, left in the school's Hall of History, Cammie reveals she has decided to leave the Academy alone. She frames her departure not as running away but as running toward: toward answers about why the Circle wants her, the truth about her father, and the mission he started. She asks that no one look for her and sneaks out of the mansion one last time.