The movie directly follows the events of The Rise of Darkrai. After the legendary Pokémon Dialga and Palkia’s battle in Alamos Town, the violent clash of time and space creates toxic byproducts that drift into an alternate dimension known as the Reverse World, a strange realm that exists parallel to our own and acts as a mirror that maintains the real world’s balance. This pollution enrages Giratina, a powerful and solitary Pokémon often called the Renegade Pokémon, who is the sole ruler of this distorted dimension.

Determined to punish those responsible, Giratina ambushes Dialga at a lake and drags the Temporal Pokémon into the Reverse World. Unbeknownst to both, a tiny, hedgehog-like green Pokémon called Shaymin, the Gratitude Pokémon, gets caught on Dialga’s tail and is pulled through the portal as well. Frightened, Shaymin unleashes a burst of its energy, creating an opening that allows both itself and Dialga to escape. Before Giratina can follow, Dialga traps it in an infinite time loop, confining the Renegade Pokémon to its own decaying world.

Shaymin is later found by the Pokémon trainer Ash Ketchum, his Pikachu, and his friends Dawn and Brock. Despite having a prickly personality and communicating telepathically, Shaymin accepts their help to reach a legendary location known as the Gracidea Garden, a high mountain meadow where it and others of its kind perform an annual migration called the Flower Bearing. However, they are not the only ones searching for Shaymin. Giratina requires Shaymin’s unique power to break Dialga’s time loop and re-enter the real world. Simultaneously, a mysterious and ambitious man named Zero, a former assistant to a scientist named Newton Graceland, also hunts Shaymin. Zero seeks to capture Giratina and use a device to steal its control over the Reverse World for himself, commanding an army of Magnemite, Magneton, and Magnezone from a high-tech mothership.

As Ash’s group travels through a region inspired by the fjords and glaciers of Norway, they are repeatedly dragged into the Reverse World. Inside this strange dimension where gravity shifts and natural laws do not apply, they meet Newton, who explains that the Reverse World absorbs imbalances from the real world, which manifest as black, poisonous clouds. To reach the Flower Garden, Ash must protect Shaymin from the pursuing Giratina and the machinations of Zero. When Shaymin touches a Gracidea flower, it transforms into its Sky Forme, a deer-like creature capable of flight.

The conflict culminates when Zero succeeds in capturing Giratina and draining most of its life force to power his vessel. As Zero attempts to sever the Reverse World’s connection to reality, he causes a massive glacier to break free and threaten the Flower Garden. The golem Pokémon Regigigas awakens to slow the glacier’s advance. Inside the Reverse World, a weakened Giratina is healed by Shaymin’s power. Ash and Shaymin work together to defeat Zero, restoring the balance between the two dimensions. In the end, Giratina is freed from its time loop and reconciles with Dialga, while Shaymin finally reaches the Gracidea Garden to complete its migration and transform the field into a sea of flowers.