Ninokuni's storyline was first exposed during the official announcement on September 2008 by developer Level-5. During the announcement, it was told that Ninokuni is a story about the life of the main character who lost his mother, and hopes to revive his mother by journeying to Ninokuni, the other world; which supposedly have the power to revive his mother.
There are no information available since the first announcement on September 2008. Fortunately, the May issue of Famitsu exposed greater details about the game's storyline. The main character is a 13-years-old boy named Oliver. He lives in the city of Hotroit with his mother, Arie, who dies suddenly. Distraught over the death of his mother, his doll, whom his mother gave him, sprung to life and introduced itself as a fairy named Shizuku. Shizuku told Oliver that by going to the Ninokuni, he may be able to revive his mother. Oliver takes on the journey and explored the world of Ninokuni.
The world of Ninokuni is told to be a "colorful" world, and the player, as Oliver, will have to venture through the world of Ninokuni, in hope of Oliver getting his mother back. Coincidentally, during his travels, the people Oliver will meet are variations of real people from Earth. For example, when Oliver and Shizuku enter Ni no Kuni, they first arrive at Goronel Kingdom, which is ruled by a lazy cat named Nyandal XIV, and the people of Goronel are apparently troubled with their ruler's laziness. Nyandal XIV is actually a cat in the real world, and apparently a neighbourhood cat named Frank.
Another example is Layla, a lady who sells milk in the real world. In Ninokuni, she was known as Kaula, the Queen of Vavalencia who has a trouble of insatiable appetite. There are also two other characters announced that parallels with Oliver's world. One is Marl, depicted as a fearless girl who loves to sing in Ninokuni, but in the real world; she is actually Shelly, a girl who is sick and homebound. A talking rat named Magooru that Oliver will meet in Ninokuni is actually Oliver's friend, Mark.
The setting will differ between both the city of Hotroit (real world), apparently a play on Detroit (the city even described itself as the "Motor City"; and Ni no Kuni itself, where most of the game will take place.