At the very beginning of Part 1 (called "The Boy Who Walks In the Light"), we meet Day, who is running from the law and wanted by the Republic.
The Republic of America doesn't really know what Day looks like though, so he's hiding in plain site with his friend Tess, who is another street child.
He's keeping an eye on his family, who live in the Lake sector of Los Angeles. Apparently there's a plague going around, and he's afraid that the authorities are going to come and mark his family's home with a big red X.
He's watching carefully—Day always checks in on his family even though it's kind of dangerous—and sees soldiers coming up to the family's home.
As he's watching the soldiers walk up, a plague victim runs out and the soldiers shoot her dead.
Day is kind of a thief, so he's brought some gifts for his family, including a used pair of goggles for his brother John—who is the only person in the family who knows that Day is still alive. Other than that, he's brought a bunch of food.
He starts thinking about his little brother, Eden, and how he's turning ten and will have to take a the Trial. That's the test that decides whether you get a higher education in the Republic or whether you join the poor… and if you fail completely, the Republic takes you away and kills you.
The soldiers go inside and are in there for a very long time. Day starts to get nervous.
Then the soldiers come out and mark the door—except it's different from the usual X. There's another line going through it, which cuts the X in half.