When the pearl of the collection - "The Beach in Pourville" by Claude Monet - disappears from the National Museum in Poznań, and Julka's aunt is falsely accused of theft - the children have to find the painting and discover the identity of the real thief. A harmonious duo begins an investigation, but Felka, a crazy 12-year-old, appears on the horizon, and with her trouble. The story of jealousy and trust is intertwined with solving a criminal puzzle.