The article is devoted to the evolution of the “plot about the Swan Knight” in the French and German epic and romance traditions. The transformation and functioning of motives borrowed from folklore and included in the narrative at a certain stage in the existence of the plot are considered. The borrowing of folklore motives is associated with the presence of a marriage theme in the plot. In the French epic tradition, which has received a book existence, the story of the Swan Knight expands due to the motive of the search for the lost spouse. In the German tradition, you can see the influence of the fairy tale plot about the snake fighter. It is concluded that the appearance of folklore motifs in the narrative entails a plot transformation, in many respects similar to the development of the corresponding plot types in folklore (for example, the transition from an archaic myth to a classic fairy tale about a fantastical spouse). At the same time, in the literary tradition, there is a partial destruction of the opposition “friend or foe”, which has a different semantics in medieval books, different from folklore. In the French tradition, the spatial opposition of the “historical” and the sacred world is of particular importance; in German monuments, the opposition “friend or foe” is interpreted from the point of view of the conformity or inconsistency of the characters’ behavior with courtly values.