Ame wo Tsugeru Hyouryuu Danchi

Kousuke and Natsume grew up together in the same apartment complex, once close enough to feel like family. By sixth grade, grief and unspoken resentment have pushed them apart, just as the building filled with their childhood memories is scheduled for demolition. During summer break, the two and several friends enter the abandoned complex, only for a sudden storm and a strange phenomenon to cut it loose from the city. When they awaken, the building is drifting alone across an endless sea. With food limited and no way to contact home, the children must learn to survive together while Kousuke and Natsume confront the memories that made saying goodbye so difficult.