Fifteen year old Dolf Wega gets to know two scientists who have invented a time machine. They have already successfully experimented with items and animals, sending them to and bringing them back from another time. Although at first reluctant, the scientists let themselves be convinced by Dolf to send him over, so that he can bring actual proof. And so he is sent to the year 1212, close to the French city Montgrivay.
Shortly after arriving he sees another boy, Leonardo Bonifacci, being beaten by bandits. Dolf helps him and gets to talk with Leonardo, although at first there is some trouble with the language. However, when it is time for Dolf to get back a enormous number of children arrive at the place and he can not get in time to the exact location he has to stand on. He sees another boy being flashed away. Now he is stuck in the Middle Age.
Leonardo and he join the group of children on their way to Jeruzalem. Het is a childrens crusade. Soon the boys notice that nothing is organised. The leaders, the monks Dom Anselmus and Dom Johannis do not take care of the kids. The spiritual leader, a simple shepherd boy called Nicolaas, only says that God will take care of them. Dolf involves himself and ensures that the children get enough to eat and drink. During the trip Dolf makes friends with several of the children, who look up to him, because he seems to know so much.
When the children get to South Germany there is an outbreak of scarlet fever and a lot of children die. Dolf tries to save as many lives by applying what little he knows about hygiene and prevention. Shortly after there is a major quarrel between Dolf and Dom Anselmus and Dolf is accused of heresy. Dolf is rescued by Dom Thaddeus who joined the group on their way. The crusade goes over the Alps, with the necessary consequences for the children’s lives.
In the mountains a group of knights steal a number of children, but Dolf is able to free them with some of his friends. According to Dom Anselmus, Nicolaas will split the sea in two when they arrive in Genua and the children will walk to Jerusalem to free the city from the Saracens.
The proclaimed king of the crusade, Carolus, has also become a friend of Dolf and tries to take care of the children he is to lead. However, in the Middle Age there is no cure for appendicitis and an operation is out of the question. Carolus dies, after having hid his pain for many days to give moral support to the other children.
Dolf does not believe a word but decides to wait. It is shortly before they arrive in Genua that Dom Johannis can not bear it any more. He confesses their plans to Dolf. It seems that Dom Anselmus is a slavetrader. He has arranged for ships to be available, so that when the sea does not split open, he can say that God provided another way. With those ships the children would be transported to North Africa, where they would be sold to the Arabs. Dolf and Leanordo find a way to stop the plans. When the children hear the truth, they are so disappointed and enraged that they attack Anselmus and tear him literally apart.
Some of the children go back to Germany led by Dom Johannis, some stay where they are and some go with Nicolaas to South Italy. On their way the group becomes smaller each day. In each city some children stay behind. Finally a small group gets to the city Brindisi in the south. The bishop of the city offers to transport the group by boat to Venice. Anyone who wants can find work there or go back to Germany after winter. During the last part of the trip Leonardo finds an aluminium box. He shows it to Dolf and in it is a note from the scientists who are searching for him. He manages to send back a message at a certain time and in the end the scientists are able to get Dolf back to his own time.