Everybody knows the story of the UGLY DUCKLING: the adventures and misadventures of a little swan that is born far from his family and is mocked and rejected by everyone. But nobody knows how the swan's egg ended up in a duck's nest. In this episode, SANDRA and FO have to resolve this mystery, having been called out by the parents of the UGLY DUCKLING, two swans who are the KING and QUEEEN of the birds. SANDRA will manage to thwart a plot to kill the "Ugly Duckling, the prince of birds. On this occasion, detective SANDRA and her assistant FO travel into the story of the "Frog Prince", having been called out by the PRINCESS. The PRINCE, who is no longer a frog thanks to the PRINCESS's kiss, has started croaking and catching flies again, after eating a piece of cake. The PRINCE, the victim of a second spell, is gradually turning back into a frog. SANDRA and FO venture into a swamp, full of dangers and mysteries, in search of the sorceress BÃRBARA, in the hope of finding an antidote.