A biology lesson’s topic at school is genetics, and Onni Sopanen begins to think that he is actually living with a wrong family, and somewhere someone - most likely Bill Gates - is missing his long lost son. Everything seems to support Onni’s doubt of his origin - his baby photos are nowhere to be found, and even his sister seems to know about it. When Onni’s friend says that a person with blue eyes cannot be the child of brown-eyed parents, it is almost certain that Onni is a changeling. What is more, there is a new boy in the class, and Onni does not get along with him. The teacher VP decides to arrange a school trip in order to improve the atmosphere in the class. During the autumn Onni and Miro forget their hostilities, and Onni finds a natural explanation for many things. Genetics prove to be useful as well, as the kids wish to have more guinea pigs.