Not everyone remembers that “Undine” or Ondine is a name from mythology, a water nymph of “The Little Mermaid” variety — one with a great gravitas attached to her love life.

With or without that knowledge, the opening “break-up” scene in Christian Petzold’s German fantasy based on that myth is dark and intense.

Johannes (Jacob Matschenz) has just summoned Undine (Paula Beer) to “our cafe,” and given her the “I’ve met someone else” speech before we meet them. She isn’t taking it well.

“You said you loved me…forever!” And, cutting to the chase, “If you leave me, I’ll have to kill you!”

And you thought YOUR break-ups had drama.

Undine is a Berlin government docent, leading tour groups through talks about the history of the city via gigantic floor models covering much of the re-united Berlin’s landscape, talks that take in architecture and geopolitics as well as its birth (in the 13th century) and many rebirths.

Undine herself is up for a rebirth. The faithless Johannes was supposed to wait in “our cafe” to continue their talk, and perhaps get more threats from the impulsive and ever-so-intense Undine. But when she returns, he’s gone. And this other fellow, Christoph (Franz Rogowski) , is ready to use “I just heard your talk” as a way of asking her out.

A couple of clumsy moves later, there’s an accident, he’s picking shards of a shattered fish tank out of her, telling her he’s an industrial diver and as she stares at the bubbling statue of such a diver from the tank, dying fish flopping on the floor all around them, it’s love at first sight.

That’s how this “Undine” goes. It’s fun to compare it to memories (or track it down on assorted streamers — Roku, for instance) of the fanciful 2009 Irish romance “Ondine,” which starred Colin Farrell. The modern German spin on this takes us into the murky depths of the rivers and reservoirs where Christoph works, checking infrastructure, welding and encountering Big Gunther, a giant catfish notorious in a water supply reservoir.

Christoph’s a natural match for a woman with (we guess) aquatic roots. Will she see that?

Petzold emphasizes the dreamy nature of the story, which can be nightmarish if you fear drowning in the dark.

Beer’s Undine is striking, given to falling for someone HARD, and clingy once she has. She is just an ordinary, single civil servant with the weight of myth hanging on her. Rogowski, who was in Petzold’s WWII drama “Transit,” has a needy, wounded Joaquin Phoenix look and vibe that he plays up here, to good effect.

But seeing a German version of this myth/fairytale reminds one to never get your hopes too high. As the Brothers Grimm and everyone who came afterwards have constantly reminded us, “fairytale romances” have a dark subtext that “happy endings” are the real myth, here.

MPA Rating: unrated, sexual situations

Cast: Paula Beer, Franz Rogowski, Maryam Zaree and Jacob Matschenz

Credits: Scripted and directed by Christian Petzold. An IFC release.

Running time: 1:30