Xavier Gens’ “Under Paris” (originally “Sous la Seine”) presents a daring premise: what if a shark lurked in the Seine River? This novelty immediately evokes memories of disaster films like “Volcano” (1997), where a volcano erupts in downtown Los Angeles, and “Deep Blue Sea” (1999), which featured a mutated Mako shark.

In “Under Paris,” the shark, named “Lilith,” is first encountered in the Great Pacific Garbage Patch by marine scientist Sophia (Bérénice Bejo) and her team. The film opens with a gripping scene of a shark attack, showcasing decent CGI and clear underwater cinematography that allows viewers to follow the action effectively.

Three years later, Sophia, now working as an aquarium guide, faces her past when Lilith appears in the Seine River. Conflicts arise among the police diving team led by Adil (Nassim Lyes), environmental activists Mika (Lea Leviant) and Ben (Nagisa Morimoto) from the “Save the Sea Collective,” and a stubborn mayor (Anne Marivin) determined to proceed with a triathlon in the Seine.

The middle portion of the film struggles with uninspiring moments, despite Bejo’s commendable performance as the protagonist grappling with past traumas. The supporting cast fails to leave a lasting impression, making it difficult to invest in their fates.

While “Under Paris” attempts to convey messages about climate crisis and marine pollution, these themes take a backseat to the creature-feature storyline reminiscent of Roland Emmerich’s cautionary environmental tales. The film’s messages, both subtle and overt, serve more as placeholders than deeply explored themes.

Where “Under Paris” shines is in its bonkers and subversive third act. Gens unleashes frenetic shark attacks, including a memorable, gory sequence set in the Catacombs. This segment channels the blood-soaked B-movie energy of Alexandre Aja’s “Piranha 3D” and the tongue-in-cheek fun of “Sharknado,” offering absurdly thrilling entertainment.

Ultimately, “Under Paris” excels when embracing its over-the-top premise and action-packed sequences. However, the film falters in its attempts at human drama and handling its ecological themes with depth, revealing a disconnect in Gens’ storytelling approach.

“Under Paris” is now available for streaming on Netflix.