Life As We Knew It explores what happens to an ordinary family in a normal American town when their world changes forever. An asteroid hits the moon, knocking it into a closer orbit. This causes all kinds of disasters that bring the world to a halt. Without shops, food, electricity, heat and water, Miranda and her family face problems together and struggle to survive.

The story begins with a brief introduction to Miranda's life. She is a normal sixteen-year-old girl who attends high school and thinks about friends and boys. The story is told in the form of her diary, allowing the reader to see her private thoughts and feelings. Everyone at her school is excited about the asteroid that will hit the moon, but when it happens it is not the exciting event they all thought it would be. Instead, the asteroid knocks the moon closer to the earth, causing tsunamis, storms and earthquakes all over the world. Miranda's mother begins to worry about the future, and takes Miranda and her younger brother Jonny out of school to help her shop for supplies. They each take a cart at the supermarket and keep going back for more until their car is full. They buy canned food, dry food, medicine, water and other supplies. Miranda thinks her mother is a little crazy for buying so much, but they will later come to be very thankful for it. Later, Miranda's older brother Matt comes home from college.

Soon shops close and food begins to run out. School is closed early for the summer because they have nothing left to feed the students. Jonny's baseball camp is still on, and he will be able to have fresh food by working at the nearby farm. When Jonny has gone, Miranda's mother introduces rationing. Miranda becomes jealous, irritable and frightened, and gets into a lot of arguments with her mother. She starts a brief relationship with a boy called Dan who swims at Miller's Pond with her, but this does not last long because Dan decides to leave the town. Miranda's friend Sammi also leaves, and her other friend Megan dies.

When Jonny comes back from camp, his father picks him up and visits them. He is heading to Colorado with his new wife Lisa, who is pregnant and wants to be near her parents. He helps them to chop wood for the winter and gives them some more food, then leaves. More problems begin to emerge, such as the huge grey ash cloud blocking out the sun, caused by volcanic eruptions. This means they cannot grow food, and things will soon get very cold. Miranda begins to realize that town is not safe anymore on her own, and has to give up high school because it is in town. She brings home textbooks for her and Jonny to home school. When the gas runs out they use the oil furnace to make sure the pipes do not freeze, and shut off the top floor of the house, moving the mattresses downstairs. Miranda's mother hurts her ankle and is forced to stay in the sunroom with the wood stove on. When it snows, they work to clear the snow from the sunroom roof so it doesn't collapse, and Matt gets the skis out so they can learn to cross-country ski in case they have to travel in the snow.

When the water runs out they turn off the oil furnace and all move into the sunroom. They are having to ration food a lot, but are managing to survive and are happy. They even manage to celebrate Christmas. However, in the new year the family gets sick with flu, and only Miranda is unaffected. She rushes to the hospital, but their mother's doctor friend, Peter, is dead. Miranda nurses her family back to health, and saves them when the wood stove backfires and fills the sunroom with smoke. They are recovering, but food is still running out. Eventually Miranda decides to sacrifice herself so that Jonny can have more food and so have a hope of surviving. She does not want her mother to have to watch her die, so she decides to walk into town and not come back. When she gets to town, she finds men giving out food in the town hall. She takes food home, and the men promise to bring more each week. The family is now through the worst of the crisis and is still alive. The world will never be the same again, but Miranda is just glad that they have each other.