Babel opens with a boy, who later becomes known as Robin, lying on the floor of his house in Canton, China. His mother, who is on the floor next to him, has just died from cholera. Robin is on the brink of death too. Just then, Professor Lovell breaks down the door of the house. He storms inside and uses a silver bar with magical properties to heal Robin. He then tells Robin that he plans to take Robin back to England with him, where Robin will receive a world-class education. Robin doesn’t know who Professor Lovell is or why he is offering to look after Robin, but Robin’s family has all been killed by cholera, and he has nowhere else to go, so he follows Professor Lovell.
Lovell brings Robin back to his estate in the Hampstead neighborhood of London. There, Robin receives round-the-clock tutoring in Latin and Greek and also speaks with Professor Lovell in Mandarin several times each week. Gradually, Robin learns that Lovell is prepping him to study at the Translation Institute at Oxford. Robin also begins to suspect that Professor Lovell is his biological father.
Years go by, and Robin eventually matriculates at Oxford. On his first day at the university, he meets another student in his cohort at the Translation Institute named Ramy, who is from India. The two become close friends. On their first day of classes, they meet the two other students in their cohort at Babel, the Translation Institute. Those students are Victoire, who is from Haiti, and Letty, who is from England. Despite some early friction, especially between Letty and Ramy, the four become inseparable.
One night when Robin is walking on campus, he sees three people apparently stealing magical silver bars from the Babel tower. One of the thieves looks remarkably like Robin. The thieves ask Robin to help, and without knowing exactly why, Robin helps them. The person who looks like Robin then tells Robin to meet him at a pub later.
When Robin goes to the pub, the person who looks like him, named Griffin, explains that he is Professor Lovell’s son, and that Robin is also Lovell’s son. That makes Griffin and Robin half-brothers. Griffin explains that he is a member of a clandestine organization known as the Hermes Society, which aims to combat England’s subjugation and colonization of other people and countries. Griffin also explains that the work of Babel is central to England’s power and its colonization efforts. He recruits Robin to join the Hermes Society, and Robin ultimately agrees to join.
As a member of the Hermes Society, Robin assists in several robberies of Babel by letting other members of the Society into the tower. Robin, Ramy, Victoire, and Letty also continue to become closer and closer during this time, and Robin loves his classes at Oxford. As school continues, Robin is torn between his desire to combat colonialism and to study what he loves. During Robin’s third year, Griffin proposes storing explosives in Robin’s room to use during a Hermes Society mission. Robin thinks the possibility of violence is a step too far and decides to quit the Hermes Society.
During Robin’s fourth year, he is in Babel when he hears an alarm go off. When he goes to check on what is happening, he finds Ramy and Victoire ensnared in a web that’s used to catch thieves. Robin discovers then that, unbeknownst to him, Ramy and Victoire have also been working for the Hermes Society. Robin works to get them free. He succeeds, but in the process, he becomes ensnared himself. The police arrive and bring Robin to see Professor Lovell, who interrogates Robin about his work with the Hermes Society. Lovell then tells Robin that as long as he promises to never work for the Hermes Society again, Lovell is prepared to overlook what has happened. Robin agrees.
The next day, Robin’s cohort receives news that they’ll travel to Canton, China, with Professor Lovell to help a British company in trade negotiations with the Chinese government. When they arrive in China, Robin sees that the representatives from the British company are rude and racist toward Chinese people. He also learns that the British company wants to continue bringing opium into China despite a ban against opium in China and the widespread harm that opium addiction is causing in the country. When Robin privately speaks with Commissioner Lin, a representative from the Chinese government, he tells Commissioner Lin that there’s no point in trying to negotiate with the British company, which ultimately only wants to subjugate and exploit Chinese people. Later that day, Commissioner Lin burns the stores of opium the Chinese government has seized from the British company, which causes the company to lose a small fortune. Representatives from the company and Professor Lovell blame Robin for what has happened, and Lovell says that he and Robin’s cohort will board a ship and return to England immediately.
On board the ship, Lovell takes Robin aside to speak with him about what happened. During that conversation, Lovell makes racist remarks about Robin’s mother. In a fit of rage, and possibly in self-defense, Robin takes a silver bar from his pocket and uses it to kill Lovell.
When Robin’s cohort finds Robin standing over Lovell’s dead body, they decide to help Robin cover up the murder. They throw Lovell’s body overboard and tell everyone else on the ship that Lovell is contagiously sick. When they return to England, Robin discovers in Lovell’s papers that the British government had always intended to use the breakdown in trade talks as a pretext to invade China. Robin thinks they must find a way to stop that invasion no matter what.
When Robin and his cohort return to Oxford, they continue to tell people that Lovell is sick. Ultimately, another professor, Professor Playfair, discovers that they are lying. He catches them in their lie during a party, and Robin, Victoire, Ramy, and Letty run to try and escape. As they run, they’re intercepted by Anthony, a former Babel student who is a member of the Hermes Society.
Anthony takes Robin and his cohort to a Hermes Society hiding place. There, Robin, Victoire, Ramy, Letty, Anthony, Griffin, and a few other members of the Hermes Society plot how to stop England from invading China. Anthony advocates for the use of nonviolent means, while Griffin is comfortable using violence. Letty ends up betraying the group and brings the police to the hiding place. When she arrives, Letty has a gun, and she shoots and kills Ramy. Robin and Victoire are captured. The police kill Anthony and all of the other members of the Hermes Society except for Griffin, who is hiding elsewhere.
Robin and Victoire are taken to cells, where they are tortured. Robin drifts in and out of sleep. At one point, he wakes up and sees Griffin, who has come to rescue him and Victoire. The three run from the prison. When they’re about to get free, they’re confronted by a Babel professor who Griffin knows, named Sterling. Griffin and Sterling take out their guns and shoot each other at the same time. Robin tries to use a silver bar to revive Griffin, but it doesn’t work, and Griffin dies.
Robin and Victoire decide they have to continue the Hermes Society’s work. They go to Babel and tell everyone there that England is planning to invade China, and that Babel scholars are complicit in the injustices of colonization. They then say that they are planning to take over Babel. Anyone who wants to join them can stay, and anyone who wants to leave should go now. Most of the students and professors leave, though a few stay behind to help Robin and Victoire.
Over the next week, Robin, Victoire, and the other members of their group occupy Babel and begin sabotaging the supplies of silver bars in the tower. That sabotage causes roads and buildings to begin to break down across England, which leads to several deaths. The English government sends the Army to seize Babel and arrest or kill Robin, Victoire, and the others with them. Just before the Army plans to seize the building, Robin comes up with a plan to make all of the silver bars in Babel self-destruct. This will take down the building, render countless silver bars in the tower and around England unusable, and destroy decades of research. All in all, it will deal a serious blow to England and prevent the country from invading China. To do it, though, Robin will have to sacrifice his own life.
Victoire and another member of the striking party decide they don’t want to die, so they leave the tower. Robin stays behind in Babel with a few other people. When the time comes, Robin and the others pile silver bars in pyramids and make them self-destruct, bringing down the tower, destroying the silver bars, and killing everyone who is still inside.