Basically, this book is one of those dystopian stories where an oppressive government is trying to control people's lives in some fundamental way. In this case, people are only allowed to have two children so that there will not be overpopulation.
In the book, Luke Garner is a third child and must therefore stay hidden all the time. Eventually, he meets another third child, Jen Talbot. Through knowing and talking to Jen, Luke comes to see how what the government is doing is wrong. Jen is killed at a protest for the rights of third children. Luke gets a fake ID and goes off to live a new life away from his family.
I am able to give you a brief summary of the book, but to further develop a more concise summary would require a little more reading.
In the book "Among the Hidden" couples are only allowed to have two children. Luke's parents have three children which is against the law. They only get enough food rations to feed the four people in their family. Luke has to be hidden away or they will take him away and put him to death. They all also punish his parents.
Luke grows up watching out the window as his siblings go in the community and have a real life. One day he believes he sees a shadow at a window. He soon learns that a girl who is hidden lives near him. They sneak and meet each other.
Luke finds out that there are other children just like him who are preparing to stage a protest. He is supposed to go but doesn't at the last minute. All the hidden children who attended are killed.
The girl's father turns out to be part of the population police who are responsible for rounding up the hidden children. However, instead of turning Luke in, he finds him an identity with a family that had lost a child. Luke leaves his family to go off and live with the family and will no longer have to be a hidden child.