20 Years has passed since Light Yamagi has been defeated. The Japanese Investigation Force has gone from hunting the biggest mass murderer in the world to investigating drug dealers, and the Japanese Police Force has moved to catching Robbers and Murderers. The world has gone back to what it was before Light Yagami found the Death Note, and people believe it will stay like that. Near has retired, his two heirs, L and Mello - named after the two other people he worked with - have taken his place.
However, in the Shinigami World, Ryuk has news that no other Shinigami, or Human, knows. Light Yagami has been reincarnated as a Shinigami. And, instead of deciding to change the world again, has sent his Death Note into the Human world for somebody to find. Then, Light would descend on the world and instruct the Human who found his Death Note in killing the criminals.
However, two other Shinigami have gotten word of this, and have joined Light. With the same interests that Ryuk first had, the three Shinigami sent their Death Notes into the human world. Light does not know of this, however, though they plan on making their intentions clear.
In the Human world, three humans found the Death Notes. One resided in the Kanto region of Japan, the one who Light sent his Death Note to, one in Germany and one in America. The three Shinigami, Light included, have descended onto Earth to seek out these Humans, and teach them the ways of the Death Note.
Ryuk, meanwhile, is now on a quest to stop Light and the other Shinigami. Ryuk has entered the Human world in search of them. He, too, has sent his Death Note into the Human World, hoping that a human will pick it up. Accounting for this, a Human in the Kanto region found it.
The Death Notes, being found very recently, have not aroused any suspicion yet, however due to the recent event of Light Yagami, they certainly will arouse suspicion soon.