chain of memories

Memory is the key.

Kingdom Hearts: Chain of Memories is the second game in the series (this is where numbering starts to get a little confusing), and picks up immediately where the first game left off. CoM was first released in Japan in 2004 on the GameBoy Advance, and was later remade for the PS2 (as Re:Chain of Memories), which was released in Japan and the US in 2007 and 2008 respectively. Alike the first game, it was remastered for the PS3 in 2013. Instead of following the battle system laid out by its predecessor, CoM follows a card based system that is executed in real time, and is slightly confusing and overwhelming, in my opinion. I much prefer the traditional attack/magic use.

In his search for Riku and the King, Sora and his companions stumble across Castle Oblivion. As soon as they do, they all begin to forget. Sora forgets who he is, his life, and his friends - his reason for being there in the first place. Naturally, at first, these are only the details, but it won't always be the case if they choose to climb higher. Eventually they'll forget themselves entirely. The castle itself uses a combat system which involves cards; cards made from Sora's memories. Sora finds himself travelling to worlds he visited in his first adventure, where all the supposed inhabitants also have hazy memories, too. As is pointed out to Sora from Marluxia, one of the Organization members:

In this place, to find is to lose, and to lose is to find.
You'll understand the meaning of that, the longer you spend here.
Sora and company decide to climb. Sora finds himself travelling to worlds he visited in his first adventure, where all the supposed inhabitants also have hazy memories, too, and continues to encounter members of the self proclaimed Organization XIII: Larxene, Axel, and Marluxia. There's also a girl. Naminé. A girl Sora has always known, whom he was friends with back on the Islands... the person he has to save from the Castle. There was never another girl. Behind the scenes, Naminé rewrites Sora's memories and erases Kairi from them, replacing them with herself, as part of the Organization's plot to gain control of Sora. Axel, however, tells Naminé to think for herself and decide whether she can continue to go through with her task. As a result, Naminé chooses to tell Sora the truth, prompting him to remember Kairi for himself. As he reaches the top floor, she gives him a choice, to which Sora makes a decision: to chose his old memories over his fake ones. Naminé smiles as Sora says they'll meet again someday, and puts him to sleep to begin the work of reconnecting the broken chains.

Meanwhile, Riku has also entered Castle Oblivion, and has encountered three other Organization members: Zexion, Vexen, and Lexaeus. Whilst Sora ascends, Riku descends the castle. Riku undergoes a similar experience to Sora, though, for his part, Riku struggles more with the darkness and accepting himself. He almost fails, until Mickey comes to his aid. After fighting with Ansem, he reclaims his heart as his own, and with DiZ and King Mickey at his side, he encounters a crossroads. He chooses to take the middle road. That is, 'the road to dawn'.