It pretty much takes place in a limbo/purgatory. Children who die without finding peace or have unresolved emotions end up there. The story slowly reveals this and explores each characters’ reasons for ending up there - often confronting their death and untangling the things which kept them from moving on. Time itself is kind of nebulous there; two of the main characters, one who starts as a sort of antagonist and the protagonist, are kind of tied together by fate with the protag being a heart donor to the antagonist. The protagonist died before and his heart was given to her, but somehow she ended up there before him even though she lived a full life (if I’m remembering right) l. It’s honestly a super beautiful story, and I think one of my favorite parts is the ending song/mv which plays at the end up each episode- it features every character, and as they move on during the series their spot in the ED is just blank with the final episode ending without any of them.