This game takes place between Kingdom Hearts and Kingdom Hearts II, where Sora, Donald and Goofy have are walking in the crevasse between dark and light. Half of the game is from Sora's point of view, and the other half is from Riku's.
Sora, Donald and Goofy finished their last adventure, now they need to find the King and Riku. When they are sleeping, Sora wakes up and begins walking around. Suddenly he sees a man in a black coat, who tells him "To find what you seek, you will loose what you hold dear". Sora wonder what he means... Suddenly he sees a castle, and wakes up his friends, and they go in. Inside they see another man, in a black coat. This man gives Sora a card, that will open a door to another world. Let the adventure begin.
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CoM begins right at the ending of Kingdom Hearts, where Sora, Donald and Goofy have left Kingdom Hearts (place) and are walking down a grassy trail, in the crevasse between dark and light. Half of the game is from Sora's point of view, and the other half Riku's.
After Sora is visited by the same creepy hooded guy at the beginning of the last game, the party comes across a strange castle called Castle Oblivion. Sensing that Riku and King Mickey are inside, they enter and instantly realize they've forgotten all their attacks and magic (surprise!) and that Jiminy Cricket's journal has been erased. Another hooded man appears and tells them a bunch of cryptic stuff including "to lose is to find, and to find is to lose" and "to forget is to remember, and to remember is to forget". He also explains that everything in Castle Oblivion, from battles to what rooms you enter, is controlled by strange cards, including the help of Donald and Goofy during battle (see Castle Oblivion for complete rules concerning the castle's cards). After giving Sora a "world card", he disappears and the party decides to climb to the top of the castle to look for Riku and the King.
As they climb the castle, they realize that they are beginning to forget things about their journey that took place in the first Kingdom Hearts game. In short, this is because the castle uses up memories in order to create cards, which are then used to change the look of the castle's rooms to match the memories. However, Sora also shockingly begins to forget about Kairi and starts to remember a second girl who loved to draw, supposedly lived on Destiny Islands and was Sora's close friend. To the player, this is alarming, as anyone who has played the first game knows there was no such girl. Additionally, the party also starts to forget about their search for Riku and Mickey.
As they head to the top floor they also come across strange hooded people, who appear to be others inside Castle Oblivion, and not illusions made from Sora's memories. They are members of Organization XIII, headed by Marluxia, who seems to be plotting an overthrow of this organization. They have lured Sora into the castle in an effort to somehow use him to their advantage.
As Sora nears the top of the castle, he remembers the second girl's name: Naminé. Through cutscenes seen only by the player, Naminé seems to be a girl being held captive by the Organization XIII members residing in the castle. She has the ability to rearrange the "links" in a person's "chain of memories", thus manipulating their memories and making them whatever she wants. Against her will, she has done this to Sora and replaced his memories of Kairi with false ones featuring herself, including a memory where Sora promises to protect her. It is through this memory that the Organization hopes to use Sora in whatever it is their plans are.
While all this stuff is going on there's a suspicious Riku running around, claiming to be the one who promised Naminé he'd protect her. As it turns out this Riku is actually a fake puppet created by Vexen, one of the Organization XIII members present in the castle. Besides fighting Sora a few times and protecting Naminé from being killed, he eventually is revealed as a fake and killed by the real Riku during his portion of the storyline... thus in the end not really mattering much.
Finally, Sora reaches the 13th and top floor of the castle and meets Naminé—to him it's a reunion, but Naminé explains what's going on and that it's actually their first time ever seeing each other. Sora tries to understand what's happening, but is still unable to push the fake promise out of his heart, using it to try and help Naminé. After killing Marluxia, and (temporarily) freeing Naminé, she offers to put Sora, Donald and Goofy into these big egg things to restore their memories to their previous state. Knowing this will erase the new positive memories of Naminé, Sora agrees to it for the sake of finding and remembering the girl who was truly "most important" to him. Before entering the egg machines, Jiminy writes one sentence into his journal: "Thank Naminé" (a quite cryptic sentence to anyone playing KH2 if they haven't played CoM).
Nomura had heard that children wanted to play Kingdom Hearts on the Game Boy Advance, so he decided to make an intermediary title to bridge Kingdom Hearts and Kingdom Hearts II. Nomura determined that a 2D Kingdom Hearts game could still be possible and could still feel like the original to gamers, and began exploring gameplay ideas.
To explain the loss of abilities gained in the previous game, Nomura had the story revolve around Sora's memories getting corrupted and implemented the card battle system to symbolize Sora's various memories. Square developed the game alongside a studio named h.a.n.d. The game was originally titled Kingdom Hearts: Lost Memories, before Nomura changed it to fit the outline of the story more. Chain of Memories was developed with a lighter tone than that of the PS2 games.
Chain of Memories was announced at the Tokyo Game Show in 2003 alongside Kingdom Hearts II. Initial details included the switch to 2D graphics, the use of cards to perform attacks, and that compressed movies would be utilized in some cut scenes, a first for a GBA game. The cut scene animations were rendered using the graphical engine of the PlayStation 2 iteration and then encoded for the Game Boy Advance by using a technology developed by Japanese company, AM3. A playable demo was first made available to the public at the 2003 Jump Festa in Japan; this demo and subsequent demos highlighted the card-based combat system. Aside from information gathered from the opening sequences, most details regarding the story were kept secret until the release.
Chain of Memories was also advertised at E3 2004 and TGS 2004 before its initial release in November 2004 in Japan.
This game would later inspire the gameplay in The World Ends With You.
Once Kingdom Hearts II had been finished, Kingdom Hearts: Chain of Memories was remade for the PlayStation 2, titled Kingdom Hearts Re:Chain of Memories. It was developed by Square Enix's fifth Product Development Division, based in Osaka, and released as the second disc of Kingdom Hearts II Final Mix+ in Japan on March 29, 2007 and as a standalone title in North America on December 2, 2008. The remake includes polygonal 3D battles and worlds using the same graphics as Kingdom Hearts. While the card-based battle system and room synthesis aspects of the gameplay stayed mostly the same, there were additions, such as the "Reaction Command" function from Kingdom Hearts II. The remake also includes new cutscenes and battles that were not in the original game. The staff felt that it stood out among other card-based RPGs.
The game made its debut in Europe and Australia with the release of the Kingdom Hearts HD 1.5 Remix for PlayStation 3 and PlayStation 4, which features remastered graphics for the game and trophy support, alongside a remaster of Kingdom Hearts Final Mix and a cutscene movie of Kingdom Hearts 358/2 Days.