Seventeen-year-old Kaz Brekker, the lieutenant for Ketterdam’s Dregs gang, is walking one night when he’s kidnapped. When he comes to, he’s in a wealthy mercher’s house and Van Eck, a prominent mercher, offers Kaz a job. A dangerous drug called jurda parem, which allows Grisha to do miraculous things like walk through walls and turn anything to gold (but which ultimately kills them) isn’t yet known to most governments. To prevent the global economy’s collapse, Van Eck wants Kaz to break into Fjerda’s Ice Court and rescue the imprisoned scientist responsible for creating jurda parem, Bo Yul-Bayur. Kaz negotiates to do the job for 30 million kruge after seeing the shocking effects of the drug.
Kaz assembles his crew. He brings on existing Dregs members Jesper, a sharpshooter; Inej, the Wraith who can perform impossible feats like climbing buildings; and Nina, a Heartrender Grisha. He then breaks Nina’s love interest Matthias out of the Hellgate prison, as Matthias is a former Fjerdan drüskelle soldier and so intimately knows Fjerda and the Ice Court. And finally, he introduces Wylan to the group. Wylan is their demolitions expert, but he’s also Van Eck’s estranged son—so he’s a hostage they can use to force Van Eck to pay up.
After rival gangs ambush Kaz’s crew at the harbor, almost keeping them from leaving Ketterdam and severely injuring Inej, Kaz discovers that Pekka Rollins is responsible for the attack. Kaz has wanted to get revenge on Rollins for years, as he blames Rollins for his brother Jordie’s death—Rollins tricked the boys and stole all their money, and soon after, Jordie died of firepox.
The crew manages to leave Ketterdam on the Ferolind and Nina heals Inej. Kaz has a crush on Inej, which Inej reciprocates—though neither will talk about it outright. Instead, Kaz only talks to Inej about the plan for her to climb up the Ice Court prison’s six-story incinerator shaft. The entire heist’s success rests on Inej successfully make the climb and dropping a rope for everyone else to climb.
Once the Ferolind reaches Fjerda’s northern coast, the six Dregs go ashore, and then Matthias leads them south. Along the way, they come across Grisha burning on pyres—the Fjerdans, and particularly the drüskelle, believe Grisha are unnatural witches and persecute them. They also encounter two Grisha working for Shu Han who are clearly on jurda parem. Both Grisha die after attacking the group. After this horrifying experience, Nina and Matthias put aside their differences. They agree to work together to betray Kaz and kill Bo Yul-Bayur so the formula for the drug dies with its maker.
Two days later, the crew reaches Djerholm, Fjerda’s capitol city and home of the Ice Court. They’ve planned their heist to coincide with Hringkälla, a festival where new drüskelle are initiated. Conveniently for the crew, a big party takes place on the White Island at the center of the Ice Court: this will provide their cover. They sneak into the prison early in the morning in a prison wagon. The prison, to the crew’s surprise, was clearly built using Grisha labor, and the Fjerdans openly display their advanced military weapons to their prisoners.
Once the crew breaks out of the holding cells, they split up. Upstairs, Nina and Kaz search cells for Bo Yul-Bayur. Nina finds cells intended to hold Grisha while Kaz finds Pekka Rollins. Kaz sets his archnemesis free, privately believing that he needs to kill Rollins at a later date to truly avenge Jordie. In the basement, Inej begins to climb the still-hot incinerator shaft. Her rubber slippers begin to melt to her feet. But just as Inej decides she can’t make the climb, it begins to rain, cooling the walls enough for her to continue. She realizes she needs a purpose beyond helping Kaz, and she decides to use the money she’ll earn from this job to buy a ship, hire a crew, and hunt slavers. She’d like to save other people from sexual slavery, which she suffered at Ketterdam’s Menagerie brothel until Kaz saved her two years ago. With this new purpose to motivate her, she completes the climb.
Everyone makes it to the roof, but Nina ended up setting off alarms when she ran into guards in the prison. Due to heightened security, the crew won’t be able to sneak deeper into the Ice Court the way they initially planned. Inej and Nina sneak in by impersonating Menagerie sex workers while Kaz and Matthias knock out two guards and steal their uniforms. Jesper and Wylan, meanwhile, must disable the main gate in the drüskelle sector, so when the crew escapes the drüskelle can’t pursue them. They manage to disable the gate and set off more alarms, as planned. Jesper also reveals that he’s secretly a Fabrikator Grisha, and he considers seeking out training to better use his power.
Inej is detained at a security checkpoint, so Nina makes her way to the White Island. There, she’s surprised to encounter Jarl Brum, the drüskelle commander who she thought was dead. But figuring that he can show or tell her where Bo Yul-Bayur is, she flirts with him, and he agrees to take her to the treasury. When they get there, though, he reveals that he knows who she is and that she’s a Grisha. Brum throws Nina in a cell—and Matthias is with him. Matthias used to see Brum as a mentor. Now, he pretends to still believe in the drüskelle cause. But once he has the information on how to find Bo Yul-Bayur, he knocks out Brum and rescues Nina.
When Matthias and Nina reach the appropriate cell, they find a teenage boy—Bo Yul-Bayur is dead and this is his son, Kuwei Yul-Bo. The Fjerdans have been forcing him to try to recreate jurda parem. Secretly an Inferni Grisha, Kuwei has been stalling. Now, he helps Nina and Matthias (who decide not to kill the boy) blow up his lab. Back outside at the very center of the Ice Court, Kaz destroys the roots of the sacred ash tree and leaps into the resulting hole with Matthias, Nina, and Kuwei. There’s an underground river below, and it carries them outside to near the road, where they’ll hopefully meet up with Jesper, Wylan, and Inej.
Meanwhile, guards take Inej back to the prison after the Menagerie’s proprietor, Tante Heleen, reveals that Inej isn’t one of her girls and is with a criminal gang. There, Inej meets up with Jesper and Wylan and reveals that she stole Tante Heleen’s trademark diamond choker. Using a diamond as a drill bit and the winch from the drüskelle gate, they create a drill to drill through the bulletproof glass and access the tank inside. Blowing a hole through the Ice Court’s walls, they drive the tank down the hill, pick up the rest of the crew and Kuwei, and head for the harbor, blowing up a bridge so the Fjerdans can’t follow.
But at the harbor, the Fjerdan navy meets them with a Grisha it plans to dose with jurda parem to apprehend the Dregs. Nina takes the last of the jurda parem Kuwei secretly has so she can save her friends, and she successfully puts the entire Fjerdan military to sleep so the crew can board the Ferolind and leave. She spends the trip home detoxing in the hold while Inej shares her new dream of catching slavers with Kaz. He asks her to stay in Ketterdam with him, but since he can’t promise to be vulnerable with her, she refuses.
When Kaz, Inej, Jesper, and Kuwei meet Van Eck on a deserted island to exchange Kuwei for the 30 million kruge, Van Eck reveals his true goals: he wants the formula for jurda parem himself: that way, he’ll be able to profit when the markets collapse. He also doesn’t care about Wylan being held hostage, as he’s disowned Wylan due to his not being able to read. Additionally, he never planned to pay up. Kuwei speaks up and reveals that actually, he's Wylan—Nina tailored Wylan to look like Kuwei; Kaz never intended to pay up, either. In the fight that follows, Van Eck’s Grisha, who are all on jurda parem, take back the trunk of money and kidnap Inej, as Van Eck realizes Kaz cares for her. Though they smash the Ferolind, Nina and the real Kuwei are safe elsewhere.
Already working on a plan to get Inej back, Kaz goes to Pekka Rollins to collect his payment for freeing Rollins from the Ice Court. Rollins gives Kaz the money he requests in exchange for Kaz’s shares in local businesses and a valuable painting. He only realizes after Kaz leaves that Kaz stole every valuable item on Rollins’s person during their conversation.
