Morris Gleitzman's Once is set in the summer of 1942 in Nazi-controlled Poland. Once follows a young Jewish boy named Felix Salinger, who has survived the Holocaust thanks to the kindness and generosity of a Catholic orphanage near him. Felix is mostly unaware of what is happening around him and is pining for his parents' return, who left Felix at the orphanage for his safety so that they could focus on their business.
Felix's life in the orphanage is challenging because of several factors, not least because of food shortages because of World War II. One day, Felix receives a carrot, which he thinks is a sign that his parents will return for him. But they do not. Instead, the Nazis visit the orphanage, which surprises Felix and many other children.
The Nazis barge into the orphanage and wreak havoc. They cause a ruckus and burn books in the orphanage's library, which worries Felix because he thinks the Nazis may target his parents because they are booksellers. As a result, Felix escapes from the orphanage to search for his parents in their home village but finds trouble (including violence) along the way.
Eventually, after finding food and shelter, Felix makes his way to his old village. There, he is shocked to see it deserted. He goes to his old house but doesn't recognize it anymore. He goes to his parent's shop but is warned to leave the village immediately because of the Nazi threat. In turn, Felix decides to try and find his parents in the city.
On his way to the city, however, Felix is threatened by Nazi soldiers and environmental hazards, like a burning house. When he sees the burning housing, Felix enters the house to see if anyone needs his help. In the home, Felix meets Zelda, a young girl his age who he rescues and takes her along with him on his journey.
Felix and Zelda go to the city but come face-to-face with the Nazis, who force them on a death march with other Jews. Felix realizes that the Nazis truly hate more than books. They hate his religion. But the two are saved from untimely deaths by Barney, a dentist who brings them into his basement to keep them safe. Barney recognizes Felix's talents as a storyteller and asks him to tell stories to his patients.
The two remain safe in Barney's basement, but their hideout is eventually discovered. Zelda finally discovers that her father is a Nazi, but the two are forced onto a train to a concentration camp. On the train, Felix finds a hole in the train, which he and Zelda jump out of, running to safety in the nearby forest.
