One of the reasons for the darker nature of the movie is because using magic comes at a price. It seems that every time you use it, it releases an invisible form of radiation, and when a person becomes over-exposed, they change into large, deformed creatures referred to as demons.
In the hopes of controlling these outbreaks, a government agency exists known as The Sorcery Management Bureau. This division hires fighters who are extremely adept at magic and fighting these evil forces. They are known as Tactical Sorcerists (but their street names are "Strait Jackets" since the job requires them to skate the edges of lunacy).
When a terrorist organization starts causing people to mutate, more demonic attacks cause the Bureau's resources to be stretched thin. So much so, that at the opening of Strait Jacket, an attack occurs in a hospital, but there are no Tactical Sorcerists available. Thankfully, Nelin Seemonz (a young Bureau employee), stumbles upon a strange (and apparently unlicensed) Sorcerists named Leiot Steinberg. While Leiot is able to defeat this monster, Nelin learns that he has a reputation for overkill, and other Tactical Sorcerists feel that he is giving other Strait Jackets a bad reputation. This is especially the opinion of one Isaac Hammond who is a by-the-books Sorcerist and doesn't appreciate Leiot's use of outlawed magic and excessive force.
As the attacks increase, Leiot is called for more and more, and when he and Isaac start having to work together, Isaac recognizes Leiot's armor and suspects the rogue Sorcerist might have killed the very person that inspired Isaac to become a Sorcerist. Considering Leiot has some dark history and personal sin that he is constantly trying to undo, this might very well be the case.
This really is an action-packed film that does a solid job of creating and exploring a whole mythos in less than an hour and a half. Between the varying magical attacks, demons and solid characters, this really is a solid anime that could be considered a classic in a few years. If you like anime at all, then you will probably enjoy Strait Jacket, especially if you like those that are darkly themed.