Light Yagumi is a high school student who has all he could want in life, but he feels the world is rotten.  A bored God of Death drops his Death Note notebook in the human realm.  When Light realizes that the Death Note really can kill anyone whose face and name is known by the one who holds it, he begins ridding the world of its worst criminals, but that power corrupts him. 

The manga is rated T+ and is meant for older teens.  If you enjoy it, I recommend the entire series, as well as Full Metal Alchemist.