Bøker av Gerda Keurentjes

The twins Josh and Tim haven't seen their neighbor Mrs. Frolic for a few days. They don't trust it. Without anyone knowing the boys decide to embark on a secret mission to search for her. They think that their neighbor, the inventor Mr. Boombang, knows more about it and try to get in touch with him. What does the inventor have in store for them and where is Mrs. Frolic?