Syaoran, a boy who wants to become an archeologist, and Sakura, a princess from the Clow Kingdom, are childhood friends with a close relationship. On a fateful night, Sakura lost all her memories as a result of a conspiracy to obtain her powers. In order to regain her memory, Syaoran seeks help from the witch, Yuuko. Yuuko tells Syaoran that he has to travel from one alternate reality to another to collect fragments of Sakura's memory. However, even if Sakura regains her memory, she will have no recollection of Syaoran. Travelling together with them is Kurogane, a warrior who was exiled from his country, and Fye, a magician who wants to escape from his King. With the help of a magical creature, Mokona, they set off on an exciting journey through time and space.

Alternate titles:
Tsubasa
Tusbasa Chronicles
Tsubasa: Reservoir Chronicle

The series frequently crosses over with XXXHOLiC (the series). Season 1 aired from 4-9-2005 to 20-15-2005. Season 2 is set to air 4-8-2006 to 9-30-2006 with season 3 set for April of 2007.
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There is also a related movie:
Tsubasa Chronicle - nendaiki Torikago no Kuni No Himegimi. The movie premiered 8-20-2005.
Summary:

In their continuing journey to find the feathers that are the fragments of Sakura's lost memory, Syaoran, Kurogane, Fai, and Sakura move through time and space with Mokona. Here, they visit the "Land of the Birdcage," a seemingly peaceful country where people and birds live together, each person having a bird companion. After a boy named Koruri confuses Syaoran and Sakura for "bodyguards" and attacks them, they learn that the king of the country possesses a mysterious power. Princess Tomoyo, Koruri, and the other oppressed citizens, having had their birds taken from them, live in hiding within the forest. In order to take back Sakura's feather, Syaoran and the others stand up against the scheming king.

The movie is adapted from the Tsubasa Manga and related to the XXHolic Movie.
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Manga information:

Alternate titles:
-RESERVoir CHRoNiCle-
Tsubasa: Reservoir Chronicle

Sakura and Syaoran from Cardcaptor Sakura are back, but they aren't alone. CLAMP's brought together characters from all their creations for this alternate universe tale about the story of Syaoran, a boy who must travel across worlds with the help of Fye, a magician fleeing his own world, Kurogane, a ninja trying to get back to his own world, and Mokona, a creature given to them them by XXXHolic’s Yuuko, to help on their journey. Together they search for feathers of Sakura's memory in order to save her life.

Originaly published in the may 2003 edition of Shonen Magazine, the manga released volume 1 on 8-12-2003 with volume 11 released on 8-17-2005.

The English volumes began releasing on 40-27-2004 with volume 8 out 1-31-2006.

liscensed in the US by FUNimation