Giratina is a large, gray, draconic Pokémon with three gold half-rings on each side of its long neck. Its head has a gold crown-like object surrounding it with two large horns pointing sideways. A thick black stripe runs vertically along its front with five red horizontal stripes. In its Altered Forme, it has two large black ghostly wings with three red tips on each that can resemble claws. These wings can change shape as if made of a liquid or gas. Its six legs are thick and have golden claws. Two golden half-circles similar to those on its neck are on each side of each leg, resembling shackles. It has a tail.

(Source: Bulbapedia)

Shaymin, the peaceful Gratitude Pok&eacute;mon, lives happily in forests and fields of flowers. But one day Shaymin is accidentally drawn into a battle between the Legendary Pok&eacute;mon Giratina and Dialga. Whisked away far from home, Shaymin is found by Ash, Dawn and Brock, who soon discover that this new Pok&eacute;mon is full of surprises! Of course Team Rocket wants to get their hands on Shaymin, and so does a mysterious man named Zero. Even Giratina is in hot pursuit of this little Pok&eacute;mon&mdash;and just what do Giratina and Shaymin have to do with the dimension known as the Reverse World?

On the other side of the world, there exists another mysterious, mirror-like dimension known as the Reverse World. In this unique space lives the legendary Pokemon Giratina, a god-like Pokemon who soon finds itself in a great struggle with the legendary Pokemon Dialga! Why are these two Pokemon fighting? And what is the secret of this Reverse World? 
Meanwhile, Satoshi and his friends are traveling when they are greeted by yet another legendary Pokemon, Shaymin. This cute and somewhat cheeky Pokemon holds the secret to the battle between the two god Pokemon, but something unexpected happens to it during its journey. In order to save both worlds, Satoshi joins forces with it to stop the fighting once and for all!

Many millennia ago, a meteor from space which threatened to destroy the world was stopped by Arceus who intervened to protect the world. While attempting to protect the world, Arceus was badly injured and Damos (&#12480;&#12514;&#12473;) took care of it. As gratidude, Arceus promised to help make the desolate land of Mich&#299;na rich and prosperous so it gave Damos the Jewel of Life (&#21629;&#12398;&#23453;&#29577;). 
Arceus let Damos borrow 16 parts of its body to create the Jewel of Life which were to be returned to it on a specific date. Arceus was betrayed when it was not returned and it vowed to judge humans when it awakens. 
Many millennia after these events, Satoshi, Hikari and Takeshi arrive in the riverside town of Mich&#299;na (&#12511;&#12481;&#12540;&#12490;) which is surrounded by nature's beauty. They decide to go the ruins of Mich&#299;na which are at the summit of a large rock. There is a large and beautiful lake at its shores and Satoshi and friends are fascinated by the lake, but suddenly, waves begin to form in the lake and tornado of water begins to wreak havoc. 
A descendant of Damos, a girl called Sheena, runs out from the ruins of Mich&#299;na. Sheena saw what was happening and she summoned Dialga and she says "Dialga, please!" and Dialga manages to stop the tornado. 
Satoshi and his friends finally get to see the ruins and Kevin and Sheena tell the old legend of Mich&#299;na to Satoshi and his friends. Moments after they explain the legend, the fighting between Dialga, Palkia and Giratina becomes fierce and the ruins of Mich&#299;na are destroyed. Arceus is awoken... 
(Source: Pocketmonsters)