This Spanish thriller is clearly influenced by the movie "Saw," but brings more intelligence and many twists to the table. As the movie begins, four people who are skilled mathematicians receive a cryptic letter that leads them to an isolated building wherein they soon find themselves trapped, with walls that start to close in on them that can only be stalled by solving a variety of complex problems. Before too long, we begin to learn more about the backgrounds of the people involved, their complex connections, and the truth behind their captor in twists and reveals that are skillfully and increasingly tantalizingly dispersed to us. The movie features a taut pace so that, while the majority of the movie takes place in a single room, it never gets stale or tiresome, and while there are a few plot holes, the characters are well-defined and the structure of the story is strong, with a fairly satisfying ending (complete with a final wink to the audience).
Also, as the other reviews here have noted, while the people recruited in the movie specialize in mathematics, the challenges they're given to solve are not based in math, but more so in logic and probability. Yet, while some have taken issue with this, I think it makes complete sense, as math is not solely a field of numbers and the like, but deals more generally with problem-solving, logistics, probabilities, strategies, connections, and so on, and all of the problems they're given fall within these realms (we even see one of the people in the room playing chess in an early scene). Secondly, and more obviously, giving these people logic-based questions and challenges allows us to try to put our own intellects and abilities to the test rather than excluding us from the battle of wits.
The movie is presented in Spanish audio with English subtitles (there is no English audio option), and the DVD comes in a standard black case with cover artwork but no chapter insert. Also, the DVD itself is frustrating to play, as the multitude of previews that play before the menu appears can be skipped on an individual basis, but you cannot bypass them all with the "Menu" button.
While not the best DVD, the movie itself, though imperfect, is strong enough to keep your attention and even warrant multiple viewings. Thanks for reading.