In 1829, Professor Richard Lovell arrives in Canton, China and uses a silver bar to revive a young boy on the brink of a cholera-induced death. His mother has already succumbed to the illness. Lovell brings the boy, who calls himself Robin Swift, to London as his ward to be educated in the craft of silver-working. Robin vigorously studies languages in preparation for an Oxford education, and correctly deduces that the stern, racist Lovell is his father. At Oxford, Robin meets Ramiz "Ramy" Rafi Mirza, an Indian student who has also come to study at Babel, Oxford's Royal Institute of Translation. The boys grow inseparable, bonding over their "outsider" status. One night, Robin stumbles across a group of people, one of whom bears a striking resemblance to him, stealing silver bars. He instinctively helps them and his doppelgänger instructs Robin to meet him at a local pub. 

On their first day at Babel, Robin and Ramy meet fellow first-year cohort members, Victoire Desgraves, a Haitian woman, and Letitia "Letty" Price, a white woman. After a slightly rocky start, the four students grow extremely close. Postgraduate student Anthony Ribben gives them a tour of the tower where they meet Professor Playfair who reveals that Babel is the only institution that admits foreign students. He speaks about the importance of translation and how silver bars help fill in the gaps between languages. Match-pairs, or words in two different languages with similar meanings, are engraved on either side of a silver bar. The bar captures the slight difference in their meanings, producing magical effects. 

Robin's doppelgänger, Griffin Lovell, is a former Babel student, Lovell's son, and Robin's half-brother. He is part of the Hermes Society, a group that opposes the British Empire’s exploitative colonialism by diverting Babel’s silver to support resistance movements and humanitarian efforts in other countries, shattering Robin’s illusions about Oxford. Griffin asks Robin to join Hermes.

In Translation Theory class, Playfair explains that no two languages ever truly align and discusses the myth of an original, or Adamic, language. Robin also takes Latin class with Professor Craft, and has tutoring sessions with Professor Chakravarti, who reveals that Griffin supposedly died on a research mission years ago. 

Robin argues with a prejudiced Lovell over England's tendency to hoard knowledge and resources, including silver, even from countries like China that provide the valuable languages that power silver bars. Robin joins the Hermes Society, regularly helping members break into Babel to steal silver and research. Meanwhile, Robin grows increasingly attached to Oxford and to Ramy, Victoire, and Letty, leaving him conflicted about his loyalty to Babel and his secret involvement with Hermes. One day, Griffin reveals that most students connected to Hermes fake their deaths, unable to morally justify their continued affiliation with Oxford. 

As the cohort's second year begins, Playfair explains that silver-working stems from the inherent untranslatability between languages. He warns that, because a perfect translation can never exist, engraving "translation" on a bar would cause a paradox, detonating the bar and rendering it useless. One day, Griffin tells Robin that Hermes's goal is to weaken the British Empire, making history more fluid and giving the world a chance to reshape the future. In a later conversation, Griffin admits that he struggles with silver-working because Lovell took him from China at an early age, severing his connection to his native tongue. Robin is a second attempt at Lovell's experiment. Soon after, Babel's magical wards catch a thief who broke into the tower, prompting Robin to worry about his own Hermes missions. He is almost caught by the wards himself which Griffin trivializes, spiking Robin's resentment toward his brother. 

Meanwhile, Victoire struggles at Oxford as she faces prejudiced, anti-Haitian rhetoric from both her advisor and Letty. From his own advisor, Chakravarti, Robin learns of Babel's resonance links, massive silver bars that power much of England's silver. Soon after, Babel professors announce that Anthony died during a research mission and the institution’s apathy over his death further disillusions the cohort. Mill workers begin regularly protesting outside Babel after a new match-pair puts many of them out of work. Lovell scorns them and the students look the other way. Robin continues to grow resentful of Griffin, and when Griffin asks him to store explosives and mocks his loyalty to Oxford, Robin leaves the Hermes Society and chooses Babel.

Meanwhile, the cohort prepares for their challenging end-of-year exams which all four pass, solidifying their future at Babel. Letty admits to Robin that she is in love with Ramy, who does not love her back. Later, the cohort stumbles across the grave of Evie Brooke, a former student whose desk Playfair once warned them to stay away from. During a late night at Babel, Robin observes two Hermes members whom he, alarmed, identifies as Ramy and Victoire. After they're caught by the wards, Robin takes the fall for them and is brought to Lovell, who surprises Robin by demanding to know how long he has worked for Hermes and if Griffin recruited him. He accuses Robin of being ungrateful, disregarding Robin's arguments that Babel is rooted in injustice. He tells Robin that Griffin killed Evie after she discovered Griffin's involvement with Hermes. Lovell offers Robin a second chance if he provides intel on Hermes and officially chooses Babel. Robin agrees.

In an interlude, readers learn that Ramy has faced prejudiced, predatory behavior from the English all his life. At Oxford, he is riddled with guilt over betraying his principles and jumps at the opportunity to join Hermes when Anthony invites him and Victoire. 

The cohort is told that they must leave for an expedition in China. Aboard the ship, Robin, Ramy, and Victoire finally address Hermes, and Ramy is outraged that Robin was involved for years without telling them, and that Robin gave Lovell information about Hermes, arguing that survival should not supersede dignity. Meanwhile, Lovell tutors the cohort in Mandarin for their role as translators in negotiations with Chinese traders, who are wary of foreign influence, while the British push for open ports and the abolition of trade restrictions on things like opium, despite the danger it poses to Chinese civilians. Lovell suggests that violence may be necessary to force open trade borders. 

Returning to his homeland of Canton weighs heavily on Robin's conscience. Robin and Ramy reconcile, acknowledging their shared complicity in colonialism. During negotiations, Robin privately admits to the Chinese Commissioner Lin Zexu that England has no intention of yielding, prompting him to burn millions of pounds’ worth of confiscated English opium; the cohort is ordered to return to Oxford. Aboard the ship, Robin finally confronts Lovell's overt racism and England's disregard for foreign lives. In a fit of rage, Robin kills his father.

Ramy, Victoire, and Letty barge in; they help Robin dispose of the body and devise a method of concealing the truth. They throw the body overboard and tell the rest of the ship that Lovell is ill. Back in England, Robin finds proof that Lovell was developing war plans. The cohort was sent to China as a pretext; the British always planned for the negotiations to fail, using the excursion to provoke Lin into declaring war. Robin shows Victoire and Ramy, and the three friends decide to commit to Hermes, unable to live in ignorance and complicity. Letty overhears and they reluctantly tell her about Hermes. She struggles to understand her friends' disillusionment with Babel, but Ramy and Victoire point out her blindness and naivety. Letty promises to stand by them, but her reaction makes it clear she does not align with Hermes—the only victim she sees here is herself. 

Back at Oxford, Playfair tries to trick Robin into sharing information about Hermes and the group flees. In doing so, they run into Anthony, who had merely faked his death, and he leads them to a secret Hermes base. Griffin arrives, arguing with Anthony over the merits of violent revolution versus peaceful protest. Griffin thinks violence is a necessity and wants to seize Babel, but the group lands on a lobbying plan to sway public opinion against a war with China. Letty goes for a walk, returning with a revolver and the police. Anthony is killed, Letty murders Ramy, and Robin and Victoire are imprisoned. 

In his cell, Robin is distraught with grief and yearns for death. Babel faculty member Sterling Jones confronts Robin about betraying Oxford, but Robin dismisses him, arguing that Oxford never showed him kindness. Griffin breaks Robin and Victoire out of jail but in a showdown with Sterling, Griffin is shot and killed. Victoire leads a grieving Robin to Griffin's apartment where they admit that they want to see Babel fall.

An interlude reveals Letty's limited childhood education despite her sharp intellect. After her brother's poor performance at Oxford and untimely death in a cart accident, Letty is admitted to Babel, where she struggles as a woman in a male-dominated arena. She finds Hermes misguided and resolves to save herself and her country as her friends, in her eyes, are too far gone to see reason.

Meanwhile, Robin and Victoire arrive at Babel where they announce their rebellion and seize control of the tower, revealing Parliament's plan to pursue an unwanted war with China. Robin argues that Parliament needs Babel’s silver and without it, the war effort collapses. Victoire exposes the Babel professors’ complicity in forcing opium into China while Robin names Lovell and Playfair as key architects of the conflict.

Robin urges the tower to go on strike to force Parliament to halt its war plans, insisting that Britain profits too much off of Babel to risk destroying it. They dismiss anyone who wishes to leave, and only a handful of students along with Professors Craft and Chakravarti, a secret Hermes member, remain. They disperse pamphlets announcing the strike and London sends word ordering them to end it. Meanwhile, an Oxford tower that had been reliant on silver-working for structural support collapses. Robin recalls the tower’s resonance bars that power England's silver and, though Victoire cautions him not to act out of grief or revenge and Chakravarti warns about civilian risk, Robin begins to destroy the resonance bars. Oxford starts to crumble.

Meanwhile, workers across the country join the strike, and Parliament sends the Army to Oxford. The troops are held back, however, by barricades led by Abel, a former protester of Babel now allied with Robin and Victoire. The strike turns deadly as the destroyed resonance bars trigger infrastructure collapse and civilian deaths nationwide, but Robin is now fully committed to Griffin's philosophy of violence.

Robin and Victoire learn that, without regularly scheduled silver-working upkeep, Westminster Bridge will soon collapse, prompting a vote on whether to continue the strike—Robin’s side narrowly wins, and the strike continues. Despite Chakravarti’s warning that the collapse will be deadly, Robin blames Parliament. Chakravarti refuses to follow a path he believes is driven by vengeance rather than principle, and he leaves the tower. Victoire is furious with Robin's thirst for violence but they still rely on one another for comfort, eventually discussing Letty and acknowledging that her betrayal was inevitable. One of the other students in the tower begins an account chronicling their side of the strike. 

Westminster Bridge falls, killing dozens and launching all-out warfare in Oxford. Others want to surrender but Robin hopes blood continues to spill. Letty arrives to negotiate and defends her betrayal. She argues that their strike won’t bring change; Parliament will simply replace them. She extends an offer to surrender and continue their work for the Empire, but Robin and Victoire refuse, insisting that they can’t support an empire that attacks their homelands. Letty insists they’ll die if they don’t comply. Robin admits to Victoire that he wants to engrave the word "translate" on silver bars which will combust, destroying the tower and killing himself in the process but also creating a ripple effect that will render much of England's silver useless. Victoire is furious but Robin, disillusioned with living, desperately wants to die in a way that will deal a serious blow to the Empire. Victoire admits that she wants to live and have a chance at happiness, and inquires whether that is selfish. Robin holds her and instructs her to be selfish, and brave.

Robin shares his plan, recognizing the massive loss and consequences for a nation dependent on Babel’s silver. He argues that while England may eventually rebuild, the disruption will buy time for the world to defend itself—fulfilling Griffin’s vision of a fluid, uncertain future where imperial momentum is finally halted. Most decide to stay and enact the plan while Victoire and another depart on amicable, albeit heartbreaking, terms. Robin gives Abel the chronicle of the strike, imploring him to spread their version of the story. 

As they utter the word "translate" over the bars, Robin studies the trembling silver, reflecting on the undeniable truth that translation is impossible and the idea of an Adamic language is naive. Translation’s true purpose is to bridge the divides between countless worlds. He thinks back to an early conversation with Ramy, when Ramy acknowledged that when he spoke, Robin listened. That, at its core, is all translation is. As the ceiling crumbles, Robin recalls the feeling of waiting for death. He felt it as a boy in Canton dying of cholera. He thinks of his mother's smile and she says his name. 

In the epilogue, Victoire flees and reflects on her childhood, when she had wrongly believed Haiti was uncivilized and barbaric, unaware that the Haitian Revolution had inspired communities worldwide. After meeting Anthony, she learned to feel pride in her Haitian heritage and native Kreyól. She has learned a tremendous amount about revolutions, violence, and victory. She does not yet know where she will go but knows that whatever shape the struggle for justice takes, she will be there to join in the fight.