The perils of dating abound if you are a mermaid. Be careful with the diver figurine you found inside a fish tank, for example. If its leg snaps off by accident, you had better glue the leg back on, otherwise something could happen to the human leg of your human industrial diver boyfriend Christoph, a man in such rough shape under your aquatic spell that he sprints after your trains when they are leaving. Concerns of a barely human kind keep swimming up from the water’s edge in director Christian Petzold’s latest film, where they are met in due course with all the ordinary contingencies: unanswered calls, seeing exes in public, a gargantuan catfish named Big Gunther, and so on. Played by Paula Beer, the title character’s hybrid being complements the hybrid structure of this fantasy-tinged urban romance, shot partially on land in Berlin and underwater, leading to a work that culls its arresting phantoms along margin lines of both genre and landscape.

Even in quasi-documentary scenes from Undine’s job as a freelance docent at the Berlin City Museum, which layer her voice onto shots of an exact scale replica of the capital in miniature, the hybrid format intrudes, as we are told that Berlin’s Humboldt Forum is a metamorphosed Hohenzollern palace, its entire urban grid a once-upon-a-time loose network of swamps. Mending dam supports in a tributary of the Rhine using an underwater soldering gun, Christoph (Franz Rogowski) comes face to face with the aforementioned Big Gunther, catching the monster on tape with the camera mounted to his wetsuit. The wetsuit camera is not so adept, though, at nabbing Christoph’s encounters with other varieties of submarine life such as ghosts, water nymphs, and ghost water nymphs. As ambiguities swirl, the viewer winds up poised at the outer margin of fantasy and reality—the zone of the myth, the place where guys up to their knees in water go to yell into the fog at mermaids and are never prepared for when one appears.