In classical literature, an undine is a female water nymph whose relationships with men are often tragic. The myth has fed into fairy tales like “The Little Mermaid,” romantic novels like Friedrich de la Motte Fouqué's 1811 “Undine,” Hollywood romcoms like “Splash” — and Christian Petzold’s moody and sensual “Undine,” where it becomes a psychological portrait that wades slowly into the supernatural.
The film, which is playing at the Kendall Square and is available on demand, reunites the stars of Petzold’s “Transit” (2019), Paula Beer and Franz Rogowski, in a lesser work but to better effect. Beer is especially affecting as the Undine of the title, a Berlin historian seen getting dumped by a cad of a boyfriend (Jacob Matschenz) in the opening scene. The shock highlights the character’s eerie disconnect, and when she reminds the cad that she’ll now have to kill him — adhering to the myth of mermaids dispatching men who dare to break up with them — we catch glimmers of an unstable mind.
Luckily for him, for Undine, and for the audience, she boomerangs to Christoph (Rogowski), a gently hunky professional diver. They meet ichthyologically cute when he approaches her at a café after attending one of her lectures and they somehow manage to knock over the establishment’s giant fish tank, deluging them in water, guppies, and seaweed. And so to bed.
The middle sections of “Undine” are its strongest, detailing the couple’s romance with an eroticism all the more powerful for its restraint. (The soundtrack sets the tone, repeatedly returning to the shimmering adagio from Bach’s Concerto in D minor.) With her ice-blue eyes and strawberry hair, Beer suggests a spookier, more cerebral Nicole Kidman, while Rogowski has the vibe of a Joaquin Phoenix after a decade or two of therapy. They make a lovely couple and the movie swoons discreetly with the passion their characters feel for each other.
So much so that the inevitable romantic roadblocks feel like contrivances, excuses for Petzold to keep us off balance as to Undine’s sanity and the murky line between reality and myth. She and the movie seem to live in two realms: The first, on land, is personified by Berlin itself, with Undine’s lectures on the city’s historical growth from marshland illustrated by scale models and maps. The second, the domain of water, is everything that is not concrete, where sirens sing their undersea songs and giant catfish rule the waves.
So is she or isn’t she? “Undine” broadens its ambiguities in its final act, which will annoy literal-minded viewers while enrapturing those with a taste for poetic uncertainty. It has to be said, though, that the film’s pace and energy flag as it moves further into the logic of folklore. Petzold is a gifted filmmaker pulled in opposite directions by politics and melodrama, and when they’re in perfect tension, as in “Barbara” (2012) and “Phoenix” (2014), a masterpiece can result. “Undine,” by contrast, is the slightest bit waterlogged.
Written and directed by Christian Petzold. Starring Paula Beer, Franz Rogowski. In German, with subtitles. At Kendall Square and available on demand. 90 minutes. Unrated (as PG-13: artfully discreet sex scenes, watery violence).