In assignment of saint Nicolas Jaap and Jacobientje go to the train station to take the train to Spain, they have to buy many oranges. Halfway Jaap surprises Jacobientje by convincing her to take the boat.
A new school year starts at the Piet Academy. The lessons get spiced up when the old Hoofdpiet is replaced by a new person. The four want to be told more about the society, but not only Lachpiet is hesitant.
Jacobientje, Guusje, Tony and Jaap get busted by Hoofdpiet on their way to the secret chamber but Guusje saves them with a lie. Once there saint Nicolas explains the four might become the successors of the teachers.
Three of the four are not confident they can assume their new positions in the society. Smartpiet tries to keep the four's attention in class but fails in doing so.
Lachpiet refuses to believe he is at the point where he needs a successor within the society. The four find an answer before the deadline despite their struggle. Guusje might not arrive on time because Lachpiet locks her in his room.
Saint Nicolas allows the unaware Hoofdpiet to punish the four for something they did not commit because the saint thinks telling the truth might imperil the society.
Lachpiet confesses to saint Nicolas he locked the door of the secret chamber. The saint is very upset because his horse Amerigo could be in great peril because of this.
A helicopter, the colossal steamboat and a submarine - all with complete crew - are send out to find Swingpiet, Bakpiet, Ruiterpiet and saint Nicolas. Tony struggles with what is more important: his education or the search.
When Lachpiet talks to himself he confesses letting Amerigo loose and the four hear him. When they confront him he confesses he did it on purpose to prevent the old society from being replaced.
All of the four are almost suspended until Guusje lies it was all her fault. When Lachpiet finds out Guusje got in trouble he confronts the saint with the truth.
