One day, fifth-grader Paul Fifth receives an agitated message from Hugo “the Shoeman” Bachmann, whose secret the boy tracked down in a previous book- Shoe # 39. Paul learns that the man’s girlfriend, the beautiful shoe saleswoman Katya, has started to complain of boredom and wants to go on a canoe trip. Since Hugo has very little experience in that area, he’d like for Paul to come along for support. The boy gladly agrees, especially when he’s allowed to invite his best friend Minna. As they paddle across beautiful Grove Lake, the children notice quite a few peculiarities and after Hugo Bachmann is unjustly fined for illegal fishing, Paul decides to get to the bottom of things.