Pokémon Adventures: Diamond and Pearl/Platinum Vol 9

From the VIZ Media edition

Finally together again, Dia, Pearl, and Platinum must battle Legendary Pokemon Dialga and Palkia, who are being controlled by Cyrus, the powerful boss of Team Galactic! Then a new Legendary Pokemon leaps into the fray! Whose side is it on...? And who will rescue Platinum's hapless bodyguards from the mysterious Distortion World they've been banished to? Plus, hang with Lopunny and meet Porygon-Z, Gallade, and... Looker?!

Source: Anime News Network Encyclopedia