Natsume discovers that the heirloom "Book of Friends" that his grandmother, Reiko, gave him possesses the names of the spirits she defeated and bound to her will. Assisted by a cat whom Natsume addresses as Nyanko-Sensei (lit. Master Little Meow), Natsume aims to return all of the names to their rightful owners.

Natsume is attacked by a youkai while retrieving a ball at school. In his haste to run away, he ends up breaking a youkai's seal, just like how he met Nyanko-sensei. Later that night, Natsume discovers a black lookalike of Nyanko-sensei, only to have him steal the Book Of Friends! As Natsume and Nyanko-sensei search for the black cat, they stumble across a gathering of youkai. The youkai plan to attack the humans for sealing their lord - the black cat Natsume's looking for. With Nyanko-sensei's help, Natsume gives back the lord's name and catches a glimpse of his fond and bittersweet memories of living among humans.

(Source: Wikipedia)

Natsume spends time playing games with youkai he has befriended. He remembers games he was left out of as a child.