Tsubasa: Reservoir Chronicle Volume One is a manga by CLAMP, and it was published in North America by Del Rey Manga in 2004. The series is rated “T” for teens 13 and up; after reading this manga volume, I would agree with this rating.
The main characters are Sakura and Syaoran, but they’re not the characters who are known from the Cardcaptor Sakura series.
Instead, Sakura is the princess of the kingdom of Clow, and Syaoran is the son of an archeaologist.
Syaoran and Sakura have grown up together; they have feelings for each other, but neither one has confessed their feelings to the other. It doesn’t help that Sakura’s older brother, King Toya, doesn’t like Syoaran.
One day, Sakura visits Syaoran at an archaeological site that he’s working at, and she touches a relief on the floor. She suddenly rises into the air and sprouts wings. The feathers from the wings suddenly spread, and Sakura falls to the ground; she is in a catatonic state.
Yukito, the high priest, says that Sakura’s memories have all vanished, and that Syaoran will need the help from the space-time witch. Yukito uses his power to send Syaoran and Sakura to the witch, who turns out to be Yuko Ichihara from xxxHoLic.
They are joined by two others who are transported to see Yuko: Kurogane and Fai D. Flowright. She sends the three of them out a mission to explore various dimensions to look for the feathers that contain Sakura’s memories; she sends a creature named Mokona to assist them on their quest.
But there is a price: Syaoran must give up what he values most. Since that is his relationship with Sakura, Yuko tells him that when Sakura’s memories are restored, her memories of him will never be found and restored. Syaoran makes this sacrifice in order to save Sakura’s life.
Tsubasa: Reservoir Chronicle contains several character crossovers from other CLAMP series; however, in most cases, they may have the same name and look like a character from another series, but their personality or circumstances may be very different. In this volume, there are crossovers from six different CLAMP series. Sakura, Syaoran, Toya, Yukito, and Tomoyo are from Cardcaptor Sakura. Fai D. Florite is from RG Veda, while Chi is from Chobits. Yuko and Watanuki come from xxxHoLic, Sorata and Arashi are from X, and Mokona is from Magic Knight Rayearth.