Set between Kingdom Hearts and Kingdom Hearts II is the year long tale ofKingdom Hearts 358/2 Days (read: 358 Days over two). The story follows Roxas, the Nobody born when Sora became a Heartless in order to free Kairi's Heart and prevent the Keyhole to Darkness from exploding. He joins Organization XIII and quickly befriends Axel and Xion, two members who, like himself, want nothing but to become whole. Both Roxas and Xion possess Keyblades, yet they don't remember their pasts. With things never being as they appear, the two struggle to seek answers. Will their pasts reveal who they used to be, and how they received their Keyblades? And what is their relation to Sora, the boy slumbering so he can recover his Memories the Organization scattered?

A week has passed since Roxas joined Organization XIII, a group made out of Nobodies – human beings who lost their Hearts. They strive to become whole again and thus wish to complete Kingdom Hearts, a force that can give them Hearts of their own. That is a task only Roxas can help them with by using his Keyblade, a legendary weapon no other member possesses. That is, until the fourteenth member, Xion, is introduced, sharing both Roxas's age and his ability to wield the Keyblade. Time and effort are put into their education and training, yet the two still manage to find time to themselves in Twilight Town, a sleepy city at the edge of Light and Darkness. There, they meet with Axel, a senior member of the Organization. The three learn the meaning of Friendship together over the salty-sweet flavor of sea-salt ice cream.

The days hardly go by idly, however. Not only do the three have to deal with the never-ending missions assigned by the Organization, they also have their own circumstances to face. Being closest to Saix, the Organization's second in command, nets Axel the ickiest of jobs. Unless he wants to become a Dusk and lose his human self, he has little choice but to oblige. Roxas sees flashes of a boy in red wherever he goes, ones resembling his memories yet with someone else in the middle of them. Fearing he's losing his mind, Roxas begins to question himself while trying to hang onto what he has. Aside from the duties assigned by Saix, he longs for the ice-cream sessions on top of the clock tower in Twilight Town with Axel and Xion. And in the meanwhile, Xion faces off against Riku, a young man deemed an Organization impostor. He wears the coat worn by all members yet isn't one himself, and seems to have a keen interest in Xion and her Keyblade. His words leave her shaken and distraught and it doesn't help at all when Saix begins to treat her as a nuisance.

The doubts slowly grow within each member of the trio. Unable to balance their own dilemmas and each other's needs, their friendship is put to the ultimate test. Can Roxas and Xion truly trust Axel, who keeps so much from them? Without remembering his past and where he came from, can Roxas know where he's headed? If he's where he belongs? Xion turns ironically to Riku, who seems to have all the answers, but what will she choose? Staying with the friends she cares for so deeply, or setting things as they should be, and put an end to Sora's sleep? Their friendship might not last their misguided decisions as the days pass by.

Kingdom Hearts: 358/2 Days (read: 358 Days over two) begins near the end of Kingdom Hearts and Kingdom Hearts II and includes within it the events of Chain of Memories. It follows Roxas, the Nobody of the Keyblade Wielder Sora, and tells the tale of his time as a member of Organization XIII. There, he meets Xion and Axel, his two best friends who together struggle with their own pasts and the Organization's plots to use them for their own ends. Remaining true to themselves is something all three strive to achieve, yet doing so might cost them more than their own lives, but also each other as the Organization strives to obtains the power of Sora's Keyblade at all costs.

Human beings are capable of losing their Hearts. What's left behind are the Body and Soul that together make up "leftover" beings called Nobodies. Organization XIII is made up of such Nobodies, specifically ones that managed to maintain their human forms. Their goal is to become whole again and for that end, they wish to complete Kingdom Hearts, a force that can grant them Hearts of their own.Roxas, the Nobody of Sora, the Keyblade Wielder, is the XIII member of the Organization. Using his Keyblade, he is to defeat the lost Hearts that became Heartless and thus create Kingdom Hearts. He quickly befriends Axel, the Organization's Number VIII and learns the meaning of friendship from him. When Axel is sent to Castle Oblivion under the Organization's orders, Roxas applies what he learned with Xion, the new XIV member of the Organization. Much like him, Xion is a wielder of the Keyblade, yet it seems the two's abilities are related.

Shortly after the plan in Castle Oblivion fails, Roxas falls into a coma, and once he wakes, Xion loses the ability to Wield her Keyblade. After spending time with Roxas she regains her Keyblade, and at that time Saix attempts to use his past relations with Axel in order to get him to spy on Xion and Roxas. The importance of that task is shown when Xion meets Riku, Sora's childhood friend who is trying to help Sora wake up from the sleep he fell into after the incident in Castle Oblivion. He seems to know about Xion's true nature and her Keyblade more than Xion herself does, and that fact along with a warning to leave the Organization leave Xion shaken and distraught. She eventually falls into a coma during which she connects to Sora's Heart and absorbs more and more of his Memories. Realizing they are not her own, however, she seeks answers and eventually finds them from Riku. She is a Replica created by the Organization who initially had no identity of her own, and not even a face. Once Sora went to sleep his Memories began leaking out into Roxas and from him into Xion, and eventually shaped her. In order for Sora to wake up both Xion and Roxas must be returned to him, but Riku gives Xion the time to reach her own decisions, refusing to force her into anything.

The two escape the Organization for a while as Xion figures things out and eventually she engages Roxas and Axel. She and Axel fight and he manages to bring her back, yet due to the violent nature of his actions, Roxas can't forgive Axel.

Now closer again to Roxas, Xion begins to take from him more than just Sora's Memories and takes his powers as well. Using the Keyblade becomes more and more difficult for Roxas to the point the Organization pits the two against each other in the hopes only one would remain, bearing all the power and is thus useful. Realizing there is no other way, Xion loses to Roxas and is absorbed by him, only to fade out of everyone's Memories. As Sora's Memories were what shaped her, giving them back means there will no longer be a 'Xion' to remember. Clinging onto those fading Memories, Roxas wishes to take on the Organization in the hopes it might bring Xion and his friendship with Axel back, yet he is intercepted by Riku who manages to defeat Roxas and brings him back to Sora. From within, Xion offers reassurance as the Memories she shared with Roxas will forever more live within Sora, but they bear nothing but empty promises as Roxas himself is slowly lost inside the one he came from.