This is not actually a movie. It's a sketch. An outline. A way for future film students to study the buddy-cop action film without being distracted from the structure by acting or clever dialogue.
B13 is a French film set in a future that extrapolates two trends--first, that the car-burning riots were symptoms of a class struggle that continued until the Parisians finally just walled off their criminal districts and abandoned them, and second, that parkour becomes a common urban sport. The latter is particularly important, because it's a big part of all the movie's chase scenes.
The basic plot is that a clean nuke has been stolen in one of the most dangerous criminal districts, and it will go off and kill everyone there unless one cop--teamed up, of course, with an edgy young native--can defuse it in 24 hours. Along the way, the plot hits all of the cliche points for one of these genre pictures: there's the family member held captive, the criminal with the heart of gold, the unbelievably deadly police officer, the tense conversation where the cop and sidekick agree to work together, and the tables turned on corrupt officials, finished with an unsupported "romance." The script seems embarrassed to be so blatant, and glosses by each of its plot points with almost a nod to the audience that yes, this is that part of the movie.
There's not necessarily anything wrong with such self-awareness, but the actors have so little chemistry with each other that it infects everything else with a kind of malaise. Say what you like about the Lethal Weapon series, perhaps the iconic example of the genre, but Glover and Gibson were fun to watch together. There's none of that rapport with anyone in District B13--maybe because, at just over an hour and twenty minutes, there's no time for it. In its quest for dizzying chase scenes and stunts, it speeds past everything else.