Callum McGregor and Persephone “Sephy” Hadley are from different worlds. She is a Cross, dark-skinned, wealthy, and privileged; he is a nought, the light-skinned lower class that was once enslaved by Crosses. Sephy’s father, Kamal, is the Home Office Minister, and she has multiple homes with servants; Callum’s parents, Meggie and Ryan, can hardly scrape by, especially after Meggie was fired by Sephy’s mother, Jasmine, three years ago. The children are forbidden from seeing each other, but for the last three years, Sephy and Callum have been secretly meeting on Sephy’s private beach. They are best friends, though Callum is very aware of the differences in their status and world views.
Sephy and Callum share their first kiss the day before school starts. Callum is one of four noughts allowed into Heathcroft High School, where Sephy is already a student; if she hadn’t helped him study, he probably wouldn’t have had the resources to pass the entrance exam. Sephy is thrilled to see her friend every day, but meeting in public turns out to be more complex than she’d expected. Before classes even start, Crosses riot outside Heathcroft, and their violence against the four nought students only stops because Sephy yells that the Crosses are acting like a bunch of “blankers”—a vile racial slur that creates a rift between Sephy and Callum. Sephy’s attempts to make Callum feel included backfire, and he decides that not speaking publicly will be best, which makes Sephy feel abandoned. When Sephy sits at the nought lunch table, she’s removed by a teacher, then beaten up by Cross girls in the bathroom; when Callum points out that noughts contributed to social and scientific development alongside Crosses, he’s sent to the headmaster’s office.
These events at school are complicated by their respective home lives. Sephy’s parents are estranged, and her mother is a lonely alcoholic. Callum’s siblings, Jude and Lynette, often argue because Lynette is unwell—after a traumatic event three years ago that Callum doesn’t understand, Lynny has been off in her own world, and she believes she is a Cross. After an especially heated argument, Ryan McGregor explains to his sons that Lynny is like this because she was beaten nearly to death by a group of nought men who found out she was dating a Cross. As a result of that argument, Lynny is injured, and she snaps out of her fantasy world; within a few weeks, she steps in front of a bus, leaving a note to Callum explaining that she killed herself. At the same time, Sephy's mother Jasmine takes sleeping pills in an attempt to end her life (though Kamal Hadley and others seem to think it was just to get attention).
Lynette’s death has a huge impact on the McGregor family, especially when the previously passive Ryan joins the Liberation Militia (L.M.), allowing Jude to come along. Callum and Sephy continue to meet, argue, and struggle to see eye-to-eye. On a day he plans to meet her at the Dundale shopping center, he learns from his family that the L.M. is planning to bomb that Cross-heavy area. He runs to Dundale and pulls Sephy out right before the bomb goes off. Meggie, who believes in Alex Luther and the pacifist route, tries to kick her husband out of the house to keep her sons safe, but she hurts her hand slapping him. Since they’re noughts, they need ID to go to the emergency room, so Callum’s and Jude’s fingerprints are taken; because of this, Jude’s prints are registered in a database and found on the scene of the Dundale bombing. Unbeknownst to Sephy, who has decided to go away to boarding school, Callum’s house is tear-gassed in the night, and Callum, Meggie, and Ryan McGregor are all arrested.
Ryan confesses to the bombing, including seven counts of murder, because the police tell him that Jude will hang if he doesn’t. He later pleads not guilty to these charges. The trial is highly publicized, and both Sephy and Callum are called to testify. Callum is asked to leave Heathcroft during the trial, and he decides never to come back—he knows that Heathcroft will never accept a nought like him. Ryan’s legal fees are paid by an anonymous benefactor whom Callum believes to be Sephy, but Sephy learns that it’s actually her mother, who still considers the McGregors friends even after firing Meggie years ago. Ryan is found guilty on all counts and sentenced to hang, but immediately before his death, the governor commutes his sentence to life in prison. Callum climbs up Sephy’s balcony that night, full of rage and pain, and they fight, then spend the night together. Ryan dies four days later trying to escape prison.
A few months pass while Callum grieves, and his unhappiness, confusion, and hatred of Crosses grows. When Jude comes out of hiding to recruit him to the L.M., Callum agrees. Sephy is leaving for boarding school, and she writes to Callum, asking him to run away with her; Callum opens the letter too late and sprints to the Hadley estate only to see Sephy's car pulling away.
Around two and a half years pass. Callum rises quickly through the L.M., but he’s uncomfortable when Jude takes over his cell. Jude shares that their next task is to kidnap Sephy Hadley, who’s home from boarding school for the first time. Callum decides to side with the L.M., luring Sephy to the beach, then locking her away as leverage against Kamal Hadley. While captive, Sephy talks to Callum, who realizes that his hatred was something he used to survive, not really part of him; he admits to Sephy that he loves her, and they make love. The L.M.’s attempt to ransom Sephy goes wrong, and Jude returns to find Callum and Sephy immediately after they have sex. Sephy escapes in the ensuing fight between Jude and Callum.
In the following four months, Sephy realizes that she’s pregnant. She hides it as long as she can, but her sister, Minerva, tells her parents. Kamal and Jasmine finally unite in their desire for Sephy to get an abortion, but she refuses. Callum comes out of hiding when he hears, and he’s captured while talking to Sephy. Callum is wrongfully charged with raping Sephy, and he is sentenced to hang. Kamal Hadley offers both Callum and Sephy the choice between Callum’s life and their unborn child’s, and both pick the child’s. Callum is hanged, and with his last breath he tells Sephy he loves her. Sephy gives birth to Callie Rose McGregor, using Callum’s last name instead of her own.
