Think of it as a kind of CLAMP in Wonderland series, where they take characters from all over the CLAMP demention and throw them together into one big kit-n-caboodle.
The story goes where childhood friends Sakura and Sayoran, from the Kingdom of Clow are swept up in a mysterious adventure involving Sakura's innate and cryptic powers. In an accident where acient ruins are excavated in the outskirts of the Clow Kingdom; Sakura's power is triggered and she ends up losing all her memories in a mystical whirl of magical feathers. Sayoran vows infront of Princess Sakura's brother King Touya and his advisor Yukito; the Moon Priest, that he will travel to the other worlds in search of Sakura's feathers.
At the same time two other people; Fai from the Celestial world, and Kurogane from a feudal world also disembark (each for different reasons) from their homeworlds and eventually end up meeting Sayoran and Sakura in modern-day Japan. Each person on a different mission all eventually realize that in some way they have the same objective. They join forces to explore vast new dementional worlds in search of Sakura's feathers.
Sayoran, Taking up the practice of his late father as an archeologist, he befriends Sakura at a young age. Strong-willed and chivalrous, he embarks on the journey to bring Sakura's memories.
(Holds the spirit of the Fire Lion)
Sakura, The princess of Clow, strong-minded and gracefull, she has the ability to find and bring out the good in people.
Fai D. Flourite, the enigmatic magician who escaped his world in search of a place to hide from his king. Willing to lend a hand to help Sayoran find Sakura's feathers as long as he dosen't get too deep into trouble. Witty and jockular wearing a perpetual smile, he holds deeper secrets than he lets on.
(Holds the spirit of the Wind Phoenix)
Kurogane, an unbeatable ultimate fighter from a rural Japan; banished from his world by Princess Tomoyo as punishment for all his useless killing. Greatly desiring to return home; he sides up with the other three and reluctantly helps to search for Sakura's feathers.
(Holds the Spirit of the Water Dragon)
With the help of a mistical creature called Mokona they travel different worlds.
So far there has been crossovers from the following CLAMP manga/anime:
Magic Knights RayEarth
many more to come...