Legend by Marie Lu is set in a distant future on Earth. Los Angeles has suffered a catastrophic flood, and a new government has come into power. A boy named, Day, one of the protagonists, is a homeless enemy of the state. Acting like a modern day Robin Hood, he steals from the rich and gives to those less fortunate (including his own family).

At the beginning of the story, Day and his friend Tess, a thirteen year old orphan, stop by Day’s old neighborhood to check on Day's family. Day checks on his family regularly by observing their house from afar. The family doesn't know he's checking. No one in the family, except his brother John, even knows that Day is still alive. John knows because Day gives him money, food, and clothing on a regular basis. During this particular check on his family, Day learns that his youngest brother, Eden, has contracted a particularly nasty strain of the plague. A large X is placed on his old home with a horizontal line slashed through it. The only thing Day has to remember his family by is a pendant that he wears.

Switching points of view, the story shifts to another of the novel’s protagonists, June Iparis. June is one of the only people to have scored a perfect score on her Trial test. The Trial test, the reader learns, is an aptitude test that all children take when they turn ten. Depending on their score, they are either sent to the best universities and high schools, or vocational schools. Participants who score very low are sent to a labor camp where they disappear forever. This is what happened to Day.

Since June has a perfect score on her test, she has been sent to Drake University. Rebelling against her ‘perfect’ status at the college, June is constantly placing herself in danger, taking risks just to prove that she is not better than anyone else. Her brother, Metias is always bailing her out of trouble as a result. This particular day, June has decided to climb a skyscraper in midtown. She is suspended from school, and her brother comes to get her. The reader learns that Metias is a captain in the army and has been June’s sole guardian since their parents’ car accident a few years earlier.

The focus shifts to Day, again, who is breaking into the hospital to try and steal plague medicine for his brother and his family. Unfortunately, when he gets to where the cure would be stored, he finds that the hospital has run out of the drug. As he is about to leave the area, a guard named Metias spots him and stops him just short of the door. Day runs, turns, and throws a knife that lodges in Metias’ shoulder. In his hurry to get away, Day’s pendant falls off. Metias picks it up and radios for backup.

Thomas, Metias’ driver, comes to June’s home to see her. He tells her that Metias has been killed. He also tells her that Commander Jameson is accelerating her training so that she can graduate early and be set on the trail of her brother’s killer, who is known only as Day. June suppresses her grief and in a few days begins investigating her brother’s killer. She spreads a rumor among the poor sectors letting it be known that there is a cheap plague medicine that will be sold at the Ten Second Place. The Ten Second Place is named for an astonishing robbery that Day committed there in only ten seconds. The robbery had taken place years before. The Ten Second Place is now an old, abandoned bank.

Day receives word and instantly knows that it is a trap. He argues with Tess who urges him to think about it. She loses the argument and Day leaves. However, he arrives early and is able to redo the wiring in the speakers at the bank so that he can speak with the agent they send without anyone knowing his actual location. June arrives and they speak this way for a while. When Day believes that it is possibly safe, he appears and starts speaking with June. However, when he sees the way she has tied her cloak around her, in military fashion, he realizes that it is a setup and he takes off. Later, Day and Tess find themselves under an old fishing pier where they see a metal plate that they can’t identify.

June knows from Day’s accent that he is originally from the Lake sector portion of the city. Back at headquarters June and Thomas watch as Commander Jameson tries to get a spy to give them information on the East coast of what used to be called the United States. Thomas seems to enjoy inflicting pain on the prisoner, and June is repulsed.

June decides that she needs to gather her own information, and she disguises herself as a homeless beggar. She heads for the Lake sector, thinking that possibly Day will be there. The reader views something called a Skiz fight, which is a street fight that onlookers are encouraged to place bets on. A girl named Tess is there. The onlookers bait her, trying to get her to jump into the fight with a larger girl named Kaede.

June steps in for the girl and ends up being a part of the fight, though unwillingly. She takes down Kaede, but Kaede is a poor loser and pulls a knife, stabbing June. June goes down and the crowd shouts insults because Kaede broke the rules. Nonetheless, June is going to die if someone doesn’t do something. Day throws a dust bomb and the combatants scatter. Tess and Day pull June to safety.

June is hurt badly. For the next few days Day and Tess take care of her, bandaging her wounds and giving her stolen antibiotics. Day and June grow closer. Unwillingly, they start to have feelings for one another. One day they share a moment and a kiss. That's when June notices the pendant that Day is wearing. Somehow he has gotten it back. June knows that this was part of the evidence that was given to her from her brother’s effects. She knows that this is Day, the person who killed her brother. She follows him when he goes to visit his family in the Lake sector, and she radios it in to Thomas.

It goes badly. June believes that it will be a bloodless arrest, but she is wrong. A gunfight erupts, and Day’s mother is shot point blank by Thomas. John and Day are both arrested. Eden, the youngest, is taken away to fight at the front of the war. June hates how things go down, but she justifies it when she thinks of her brother.

June asks to be assigned to interrogate Day. She asks him directly about the day at the hospital with Metias and tells him that Metias was her brother. She also tells him that Metias is dead and that it is Day’s fault. Shockingly, Day admits to many of the accusations that day, but he vehemently denies killing Metias. He tells her that his knife struck Metias in the shoulder, not a fatal blow. Despite her resolution to remain hardened toward Day, June finds herself believing him. She tells Day that Tess is safe. Then, she tearfully says she didn’t know that they would kill his mother. As Day is taken back to his cell, he walks past a body bag lying in a room. He has the fleeting thought that perhaps the plague is an instrument of control being used by the government to suppress its citizens and keep them weak.

A celebration is thrown in June’s honor for capturing Day, one of the most notorious criminals of the state. A ball is thrown that night, and June attends with Thomas as her escort. After it is over, Thomas tries to kiss her; but, June pushes him away. She can’t forget how he interrogated the prisoner earlier in the week, and she realizes that he is a violent person who enjoys making others suffer. She wants nothing to do with him.

Curiosity leads her to hack into the records division where she learns that (despite what everyone was told) Day also scored a perfect score on the Trial test. For some reason the Republic had determined him dangerous enough to be gotten rid of. Why did he get a death sentence and she was given the world? It makes no sense.

Day learns that he is to be sentenced to death. The city erupts in violent protests when the news is made public. Jameson, commander of the army, takes to torturing Day, though Day doesn’t know why. The woman doesn’t even ask him any questions. She takes Day and ties him out in the hot sun for several days. June sneaks out to see him and brings him water and food. She sits with him as long as she can, and asks him pointed questions about his Trial time and what life was like for him before he took his test.

Day tells her many things, but the most shocking is his accusation that the children who fail the Trial are not sent to labor camps, but are tested on, or outright killed. He says that he knows that his records are wrong because after his test he was taken to a lab where they tried to experiment on him. He managed to escape. He also tells her of his suspicions about the plague epidemic and it stuns June. Later that day, June hears Jameson tell Thomas to gun down all of the protestors in midtown who are marching peacefully to protest Day’s arrest. She goes to Thomas to try to get him to understand how wrong that would be, but he smacks her, then shoves her to the ground.

Day is still baking in the sun and receives a visit from Thomas, who continues to torture him. He asks Day for information concerning the Patriots, a resistance group from the defecting side of the US (called the Colonies). Day doesn’t recognize any of the names that Thomas throws out at him, except the name of the girl from the Skiz fight with June. He doesn’t tell Thomas anything and the man leaves.

Later, Thomas tries to apologize to June for the way he treated her, but she tells him to go away. She pulls up surveillance footage from the day her brother was killed. She is able to see that the knife that Day threw did, indeed, go into Metias’ shoulder. That means that it must’ve been Thomas who killed Metias, and that for whatever reason, Commander Jameson knew about it and helped in the cover up. June runs to Day and tells him what she’s found out. They reconcile.

Back at her home June pulls out Metias’ journals and starts reading them for any sort of information that she can use to accuse Thomas and Jameson of collusion. Metias has used his journal to leave a coded message that only June would have been able to decipher. It tells her to go to a blog where Metias has left a video message for her stating that he believes that their parents were actually killed by the government because their dad realized that the Republic was using the plague as a biological culling instrument, destroying the ‘weak’ and ‘inferior’ genes from the population. He warns her that Thomas knows about the plague and condones it. Fearful, June sneaks out of the house and goes to free Day.

Day and June go to find Kaede, whom Day tells June is a member of the Patriots. They find Kaede and tell her about a plan they have formulated. Day will go back to his cell and when both he and his brother John are led to be executed, Kaede will create a diversion that will create enough confusion that they will be able to get away. Day goes back to his cell. Unfortunately, at the last minute they learn that Jameson is moving up the date of Day’s execution.

Desperate, June steals an electro-bomb which prevents guns from being discharged and gives it to Kaede. Thomas discovers that she’s taken the electro-bomb and comes after her. Before he can do anything the electro-bomb goes off out in the square where people are gathering to watch the execution. People begin to panic and a riot ensues.

June takes advantage and runs to free Day and John. When she gets to their cell she finds that Day can’t walk because of the beating he’s taken. As the soldiers discover their escape attempt, John stays behind to slow them down, allowing June to get Day out. They are taken by the Patriots to their basecamp where they learn that Las Vegas has become a Colonies headquarters and that Tess is there. The book ends with the Patriots, June and Day as their new leaders, determining that they need to gather enough forces and weapons to go and rescue Eden from the war front.