In the story, the water-spirit Undine marries a knight, Huldebrand, so that she might gain a soul. As one might expect, when one steps into the realm of faerie, they step into the perilous realm. Huldebrand finds himself in over his head and begins to have second thoughts about the marriage, which lead to his end.
Undine has parallels to that legendary tale and Disney movie, The Little Mermaid. Both have men that fall for water-spirits but have forces set against their union. They both have their roots in the French folk-tale of Melusine. In that tale, a mermaid/water-spirit marries a knight on the condition that he won’t ever see her on Saturdays – the day she takes on her mermaid shape.
If you want hard copies of the book, you can grab the paperback here. If you want to learn more about Undines as a topic, this is a good place to start. If you want a film, you can watch 2010s Ondine with Colin Farrell.