The Platinum Episode is a post-game campaign within Temporal Diamond and Spatial Pearl that revolves around Cyrus, Giratina, and the Ultra Beasts.
Act I: Turnback Cave
The player wakes up in their home within Twinleaf Town after defeating the Elite Four and Cynthia. However, as they step outside, two Team Galactic grunts and a man with dark blue hair approach the player. The man identifies himself as Mercury, a former Team Galactic commander. He challenges the player to a battle, and once defeated, tells them about his research in Alola with the Aether Foundation that Cyrus sent him to perform. Mercury talks about the Ultra Beasts, and how their energies seem to be coming from Turnback Cave, where Cyrus's forces are.
Upon arriving at the cave, Team Galactic grunts are inside, searching for the Lustrous/Adamant Orb and the Griseous Orb. As the player and Mercury proceed through the cave, battling Team Galactic grunts, they come across a cave painting that shows a shadowy entity of sorts battling several monsters which Mercury identifies as the Ultra Beasts. Eventually, Cyrus is confronted at the heart of the cave, where he has the two orbs in hand. He taunts the player and Mercury before leaving for the Spatia/Cronos Ruins south of Sandgem Town.
Act II: The Spatia/Cronos Ruins
The player, Mercury, and the two grunts arrive at the Spatia/Cronos Ruins, which is occupied by Team Galactic grunts. After solving perilous puzzles, defeating the grunts, and confronting Mars and Jupiter, the Chamber of Palkia/Dialga is opened up. Inside the chamber, Cyrus orders Charon to use his newly-invented device to take Dialga/Palkia out of its Poke Ball/PC Box, as well as the Adamant/Lustrous Orb, while Cyrus summons in Palkia/Dialga with the Lustrous/Adamant Orb. The Griseous Orb is merely tossed aside, as Cyrus only needed it to get into the ruins in the first place. The two dragons stand in the chamber, bound to Cyrus due to him having created a new Red Chain. Cyrus taunts Mercury, mentioning that everything he did, he did for Mercury, as he was his son and wanted him to live in a world without spirit. Mercury mentions that spirit is what makes humans and Pokemon what they are, but Cyrus refuses to listen.
As Cyrus orders Dialga and Palkia to create his new universe, the skies of Sinnoh darken. Gateways begin opening up, with the Ultra Beasts coming through, as they hijacked the dimensional rift Cyrus forced the two dragons to create. Cyrus is incredibly angry that they decided to ruin his plans, and knows that they will destroy the world and turn it into something far worse than what he wanted. All seems lost, until a shadowy portal opens up in the ruins. Giratina emerges from this gateway, and manages to close the gateways, redirecting the Ultra Beasts' gateways to the Distortion World, freeing Dialga and Palkia and allowing the legendary dragon owned by the player to return to their Poke Ball/PC Box, while the other dragon simply gives the player the Lustrous/Adamant Orb before disappearing. Giratina then takes Cyrus into the Distortion World, leaving the player and Mercury to follow him.
Act III: The Distortion World
The Distortion World is now full of Ultra Beasts, as well as Pokemon that were sucked in through the portals. Mercury tells the player to find Giratina and Cyrus, as he believes that Giratina was the Pokemon depicted fighting the Ultra Beasts in Turnback Cave's cave painting. As the two of you shut down Ultra Beast gateways, Giratina's roars can be heard all across the Distortion World.
Eventually, after all the gateways are closed, Mercury leads the player to the Altar of Giratina, where Cyrus has managed to take control of Giratina using the Griseous Orb. He sends Giratina to attack the player and Mercury, but the player manages to catch it. Afterwards, Mercury teaches Giratina Void Pulse, and Cyrus challenges the player to a battle, furious that his plans were ruined once again. After his defeat, he decides to remain in the Distortion World, as it was the world he was seeking the whole time.
Mercury and the player both leave the Distortion World. Mercury battles the player one last time before giving them the Staraptite and a ticket for the ship to the Battle Zone, where he lives. If the player visits him there again, he upgrades their Mega Ring into a Power Ring, allowing for Z-Moves to be used.