Kingdom Hearts: Melody of Memory, like many of the side projects released outside of the main series, is an important part of the franchise's story as a whole and actually sets up the next game in the series when it's all said and done. The rhythm action game is a melodic trip down memory lane as various tracks from across the games are morphed into a whole new kind of gameplay not seen in this franchise before. Even with this major shift away from the usual action standards of the main games, Melody of Memory is hiding some major story bits.
Warning! Full story spoilers from Kingdom Hearts: Memory of Melody to follow! Memory of Melody's title and cover art tease the entire purpose of the game in the story of the series as the game has Kairi re-telling the franchise in her voice while Ansem, Even, and Ienzo search through her memories for where Sora disappeared to following the events of Kingdom Hearts 3.
Eventually Kairi finds herself in the empty space of the Final World that Sora found himself in during Kingdom Hearts 3. A visage of Xehanort appears before her and tells her that Sora has gone to a place that is outside both the realms of light and darkness. It's a place where their voices can't reach. After she tells Ansem about this, he hypothesizes that there is a reality outside of their own reality -- an unreality, or as he clarifies, a fictional world.
Upon reuniting with Riku, Riku reveals that he's been dreaming about a large city with tall buildings. Ansem then further concludes that the three realms of their world of light, darkness and in between (along with memories, dreams, and data) can be traveled to, but if Sora's in a place where their voices can't reach, then he must be outside of their reality and within this fictional realm.
The Fairy Godmother, upon Yen Sid's request, then appears and reveals that Riku and Kairi are two of the three keys needed to rescue Sora. Kairi's memories and Riku's dreams are already set, but the third key needs to be established. Taking Riku and Kairi to the Final World, the Fairy Godmother introduces them to a girl that has lost her form and has become one of the clusters of stars that Sora had come across in the Final World during the events of Kingdom Hearts 3.
As it turns out, this girl (who remains unnamed) is the final key needed to save Sora. She's from this other reality, and it's stated that her form was "taken" from her, but she managed to find her way to this reality. If she goes back to her world, her heart will be stolen. The Fairy Godmother can't help with her magic yet, but there's a way to help her as Riku can use his power of waking to get to her world so she can open a gate on her side.
The mysterious girl then posits that the city of tall buildings Riku has been dreaming of is something named "Quadratum," and thus Riku uses his power of waking to head to the other side. Kairi stays behind as she knows she's not ready yet, and thus asks Yen Side if she can train under Aqua to complete the rest of her mastery training.
With Riku now going to this other side, Kairi now training under Aqua, and Donald and Goofy tasked with updating everyone on Sora's wherabouts, Mickey has been assigned with heading to Scala ad Caelum (the final world in Kingdom Hearts 3) to learn more about the ancient masters as they seem to have once opened up a gate to this other side.
It's currently unclear of whether or not this is teasing Kingdom Hearts 4 or anything like that, but this is certainly setting the stage for the next entry in the series teased by the secret movie in Kingdom Hearts 3 that saw both Sora and Riku in a mysterious city full of buildings. But what do you think of this set up? Let us know your thoughts in the comments or you can even reach out to me directly about all things animated and other cool stuff @Valdezology on Twitter! You can also read our full review of Kingdom Hearts: Melody of Memory at the link here.