Foggy World is a fictional country, one of the many countries visited by the main characters of Tsubasa: Reservoir Chronicle.
After leaving Koryo Country, the group lands in Foggy World. Foggy World at first glance seems to be uninhabited by people and consists of a giant lake surrounded by jungle, but Syaoran asks Mokona anyway if Mokona feels a feather. Mokona answers that Mokona can't be sure it is the feather's wave, but there is something in the giant lake.
Syaoran says that he'll check it out while Fay and Kurogane can check the surrounding area for any people. Sakura starts to interrupt, saying that she should be the one to go down into the lake, but she falls asleep and is left by a campfire that they build later. When she awakes, Syaoran (soaked) is pulling his shirt back on behind a tree. Sakura runs up to him and asks if he's been in the lake the entire time. Syaoran answers yes, and that it would be too dangerous for her to go down into the lake because she could fall asleep. Sakura responds that she wanted to do it, and then asks if she could at least tend the fire while he rests. Syaoran agrees, knowing that she wanted to be of a help in some way because that is her nature.
Sakura starts talking about the memory she regained in Koryo Country, and that she seemed so happy to an empty chair at her birthday. Syaoran's memory reveals that he had been sitting there, and that later Sakura had even given him a birthday of his own because Syaoran had been found on the streets only a while ago and didn't have any memory before then. Sakura gave him the same birthday as her own, April 1st.
The story switches to Fay and Kurogane out walking. Fay begins suspecting that no people live in this world, and Kurogane says that it's very scary. Fay tells him not to worry and he'll hold his hand, and Kurogane answers that that would make him so happy. Kurogane then gets mad at Mokona who had been copying his voice to talk to Fay, and Fay praises Mokona on how much Mokona sounded like "Kuro-puu." Mokona responds that it is one of Mokona's 108 secret techniques, Fay asks what the other 107 are, but Mokona tells him that they are all a secret.
Fay and Kurogane then decide to head back towards Syaoran and Sakura. Syaoran and Sakura's conversation is cut short when the lake next to them starts glowing. Syaoran shouts for Sakura to hide, sheds his boots, gloves, and cloak, and then dives into the lake. Sakura trips when running after him, and is pulled into a memory. It is when she first meets Fujitaka and his son, King Clow tells her that the boy had been through a lot of hardship and his heart was still frozen. But his heart would melt if he could see a smile like Sakura's that is as warm as spring sunshine. King Clow asks if she would like to be the boy's friend, and the young Sakura nods ecstatically. The actual Sakura is then released from the memory.
Syaoran, while swimming downwards, recalls the same memory, and is revealed to be the little boy that had no memories. When he came back to Fujitaka's house after the birthday party, he told his adoptive father that the princess smiled at him. She didn't look at him with pity, but smiled at him really warmly. Syaoran continues swimming downwards and sees a tiny town. He looks up to see the source of the glowing, a giant fish. Syaoran makes the connection that the fish must serve the same purpose as the sun to the town.
A scale drops from the fish and Syaoran grabs it, reminded of an old saying of Fujitaka's. When he surfaces again, Mokona comes up to him saying, "Sakura is...!" Syaoran responds, "SAKURA IS WHAT?!?" Mokona answers cutely that she's asleep. Syaoran crawls up to her, and sighs relievedly. Mokona says that it was one of Mokona's secret techniques, "Super Dramatization!" Fay tells Syaoran that there are probably going to be more instances like this one, and to not get so worked up over it. Fay also tells him that Syaoran is allowed to smile, and everyone would be made happier by it, including Sakura.
Syaoran is asked about the lake, he answers that there was a town down there, and shows off the glowing scale he had dropped. Mokona waddles over to it, and says that the scale had been the power it had been sensing, and there was no feather on Foggy World. Kurogane gets mad saying that then this dimension was for nothing. Sakura then wakes up, and shouts that, "Syaoran-kun is still in the lake!" She starts running as fast as she could towards the edge of the lake, but Syaoran catches up to her and stops her before she is able to dive into the lake. Sakura answers that she's so glad that he's okay. Mokona says it's time to go, and the four dimension travelers leave Foggy World.