Cousin Herbert's obsession with toys has inspired Hansje to swap more than just comics in the Familie Knots swap shop. Herbert shows a different side to himself when he visits in a professional function with his colleague De Wit. They are investigating a possible case of fraud by Tante Til. Arend Vogel's tutoring also comes under scrutinizing. It turns out the tax forms were not posted in time. Now Tante Til needs her amateur sleuth husband Frederik more than ever, but unfortunately he is out.
De Belastingdienst heeft geen aangifteformulier ontvangen van de familie. Dus wordt er een inspecteur èn ambtenaar Neef Herbert op afgestuurd. Neef zit tussen twee vuren: Moet hij zijn plicht doen of zijn familie helpen?
