Light Yagami is a college student with a naive outlook on the world: he thinks that if he pursues a career in Law that he will actually be able to make a difference. One night he witnesses something that shatters his beliefs and just a few moments later he finds the Death Note. It's a notebook that belongs to a God of Death (think Grim Reaper) named Ryuk and there's a shitload of rules to it, but simply put: you write a person's name in the book and they fucking die.
That's a huge oversimplification of the story, but I don't want to give too much away, cause the biggest thrill is discovering all the new rules and stuff as the it happens. Some of this stuff is so goddamn silly I was howling with laughter, but it was great! I really can't recommend this movie enough. I had a blast.
Usually I really dislike the use of CGI mixed with live action, but I thought the way Ryuk looked was really cool. At first sight it looked kinda silly, but I quickly grew to like it a lot. Highly recommended.
Update: It's been a few months now and I've watched this movie at least 20 times. I can't quite put my finger on it, but I love this movie! I read the manga also and it was really good for the first 6 or 7 books then it kinda died a long boring death. Which makes the movie even more impressive. The blu-rays for I & II look fucking amazing. You should buy them!