The Impossible Kiss is the kiss of Tarabas. When Romualdo is turned into stone, Fantaghirò seeks out the help of the Dark Witch, who tells her that the spell can only be broken by a kiss from the person who had cast it in the first place.
This means that Fantaghirò needs to get a kiss from Tarabas, so she starts searching for him. She happens to cross paths with him in the forest, unaware that he is the man she is searching for. Tarabas is intrigued by her and almost kisses her when she is asleep. When he is about to he transforms into a hideous beast.
Xellesia explains to Tarabas that the transformation is part of the conditions of the Impossible Kiss. He is cursed, and whenever he attempts to kiss anyone he has feelings for, he will turn into a beast and rip them limb from limb. Tarabas tests this by kissing Xellesia. He does not transform, which he takes as evidence there is no love between them.
Fantaghirò manages to get the Impossible Kiss from Tarabas in exchange for becoming his wife and living with him in the underground lair. Tarabas shackles himself to the wall when Fantaghirò approaches; he transforms and she kisses him, and the Kiss manifests itself as a ball of coloured light that the Dark Witch captures in an orb. The Dark Witch promises to bring the Kiss to Romualdo but she lied, and Fantaghirò is forced to chase after the Dark Witch and get the Kiss back. By the time Fantaghirò reaches Romualdo, the Kiss has already lost its power and is useless.
Fantaghirò returns to Tarabas despite the Kiss not working, because she promised. Tarabas realizes the depth of her love for Romualdo and breaks their agreement. Fantaghirò tries to claim another Kiss but only the first would work. Fantaghirò then suggests that he kiss Smeralda, who he does have affectionate feelings for. He does and Fantaghirò captures the new Impossible Kiss, using it to restore Romualdo to life.