Bouke tells Tom and Mirjam about the legend of "Flying Volckert" who used to harass everyone in the village in the past. Together with their dog Snuf, Tom and Mirjam decide to investigate the story. Near the swamp they encounter gamekeeper Grevendonk who's caught in a bear trap and swears it was Flying Volckert. When inexplicable things start happening in the village, the village people decide to take matters into their own hands and together with the local police they start looking in the swamp.

Tom goes along with Miriam and Snuf one week stay with their uncle and their cousin Hans Jaap. Near the house of Uncle Hans was a haunted castle.

Adolescent Tom lost his parents and home in the Nazi bombing of Rotterdam. The orphan is taken in on uncle Tjerp's Friesian farm. Old farmhand Bauke gets him a (German!) shepherd puppy, even grumpy Tjerp can't resist their instant bond. As Tjerp feared, the boy takes grave risks wanting to revenge his parents on the German occupation garrison. Finding Dutch RAF gunner Haaksma starts a series of dangerous adventures.