Without ever actually seeing the 25-episode TV series, it was somewhat awkward going into "Blue Exorcist The Movie" blind. However, the film is fairly easy to understand even if you're not familiar with the manga the anime is based on or the series in general. There are a few things to keep in mind to make your viewing experience a little easier though.
The film revolves around half-demon twin brothers Rin and Yukio Okumura. Rin is rather spontaneous and attempts to solve everything in the moment while Yukio is very by-the-books, a bookworm, and a stickler for doing everything according to procedure. The two were raised by a priest named Shiro Fujimoto, who was actually Satan. Rin and Yukio aspire to be Exorcists and are both enrolled in the True Cross Academy. The "Blue Exorcist" world is divided into two realms; Assiah, the realm of the humans and Gehenna, the realm of demons. Demons have found a way to invade the human world and it's up to the Exorcists to stop them.
"Blue Exorcist The Movie" begins with Yukio, Rin, and Shiemi attempting to exorcise a phantom train while it's moving. Shiemi is a young girl who has potential as an Exorcist, but has this creepy green booger thing named Nee on her shoulder that wears a leaf hat and has plant related powers. The phantom train sequence in general is extremely fun to watch. There's actually another scene near the end of the film that's similar that acts like a bookend to the opening scene. The action is fast paced, the animation fluid and crisp, and the film always finds a way to hold your attention.
A very important factor to the film is a story that Shiro used to tell Rin and Yukio when they were young. The story is about a weak demon who is fed by a boy in a small village to get its strength back. When the demon is better, everyone in the village becomes carefree and ignores their work. An Exorcist eventually seals the demon away, but it's too late. The village had withered away. After the phantom train is subdued, Rin rescues a young boy in the wreckage without realizing the boy is a demon. Meanwhile a festival that is only held every 11 years is in the midst of its celebration while the protective barriers around the city to keep the demons from invading are repaired.
The true form of the phantom train is covered in eyeballs and dozens of hands, all the while resembling the demons in hell in "Berserk." There are so many bizarre creatures in this that seem like they require a film of their own to fully explain their story. There are creatures referred to as MOLB, which is short for Monster Of Liquid Balloon and is basically a large water balloon monster. There are a ton of tiny telepathic communicating animals assisting the Exorcists. In addition to Nee, there's a green gerbil, a black cat with two tails that grows at will, and a fox. Why does Rin have a tail while Yukio doesn't? And what's up with Rin's sword causing a blue aura around him whenever he unsheaths it? While you can understand the film without seeing the TV series to a certain extent, the film still leaves you with a handful of questions.
Even though you may be a little lost when it comes to the backstory of some of the creatures and characters, "Blue Exorcist The Movie" is extremely captivating and is beautifully animated. Obviously you should check out the TV series before seeing the film to fully understand everything, but even so "Blue Exorcist The Movie" has shades of "Fullmetal Alchemist" and "Berserk" and will leave you with the urge to view the television series or even revisit it again.