250 years in the future, all will be lost unless the fifth element is found.
Why it is hard sci-fi
Soft. Bessons opera is built for spectacle and the science backbone is a magical fifth element that defeats a planet-sized weapon. The films one consistent piece of worldbuilding is the air-traffic system over a vertical New York, where the films pacing depends on three-dimensional traffic rules with cops, fines, and a working dispatch. That part is the only place the film does engineering.
Science inside it
No working physics. The film treats genetic reconstruction as a single-machine procedure that produces a fully formed adult from a glove fragment, which is a fiction it makes no attempt to defend. The vertical urban infrastructure is the films hardest piece of worldbuilding and is shown working under load.