A Death Note from a Death God
The manga is about a notebook dropped on Earth by a Shinigami, or a Death God. Light Yagami is the one to find the book and discover its supernaturel power to kill the person you write the name on it. It is the DEATH NOTE.
The lost notebook
Ryûk, a Shinigami (God Death) says it’s been five days since he dropped his notebook (or Death Note) to the Human realm and should be about time for him to leave to the Human realm and take the book take back to the Shinigami realm.
Light finds the book
Light Yugami is an extraordinary smart student. He is seen bored in class, then sundenly he sees a book drop from the sky to land on the floor. After school ends, Light walks past the place the book dropped at and sees the book with a title written as Death Note.
He opens it, reads the instruction on it, and laughs at "The human whose name is written in this note, shall die", thinking it stupid, sick and juvenile.
The Power of Death Note
Light takes the book and bring it to his home. Once in his room he decided to read the "How to use it" and discovered that writing down the real name of someone you have his face in mind, he will die after 40 seconds!
Once he touched the book he is able to see and listen to the Shinigami, Ryûk in that case, when for all the other humans he remains invisible.