How to Catch Dialga and Palkia in Pokémon Platinum

Talk to Cynthia's grandmother in Celestic Town. She will read a book about Legendary Pokémon to you.

Buy Ultra Balls. There's no Poké Mart in Celestic Town, so you'll have to buy your items from an elderly couple's house in the northwest of town. The man has Dusk Balls and the woman has Ultra Balls.

Find the Adamant and Lustrous Orbs. They're hidden in a chamber in Mt. Coronet near the upper end of the waterfall. You'll need HM Waterfall to reach it.

Head to Spear Pillar. After you have the Adamant and Lustrous Orbs, you'll be able to find Dialga and Palkia's rifts at the top of the mountain.

Save your game. If you accidentally defeat the legendary or are beaten, you'll want a save point to return to.

Enter the blue portal and catch Dialga. You'll enter battle with Dialga when you enter the portal, so be ready. Dialga is a level 70 Steel-Dragon-Type Pokémon with the signature move Roar of Time. That's an insanely powerful move that will probably cause your Pokémon to faint. Use Ultra Balls to catch Dialga quickly. You may need to use a few Ultra Balls to successfully catch Dialga. When you catch Dialga, you'll be transported out of the blue rift and back into Spear Pillar. You'll still see the blue rift, but it's empty.

Save your game. After you successfully catch Dialga, you'll want to save your game so you don't have to catch it again if you end up reloading your game again later.

Leave the area and re-enter. You'll want Palkia's portal to appear, so you'll need to leave the area and come back in. When you re-enter Spear Pillar, you'll see a pink portal. You can save the game again if you want to, but you saved your game immediately after catching Dialga, so if you reload your game, you'll just need to run out and run back into Spear Pillar.

Enter the pink portal and catch Palkia. You'll enter a battle with Palkia when you enter the portal, so be ready. Palkiais level 70 and is a Water-Dragon-Type Pokémon with the signature move Spacial Rend. That's an insanely powerful move that will probably cause your Pokémon to faint. Use Ultra Balls to catch Palkia quickly. You may need to use a few Ultra Balls to successfully catch Palkia. When you catch Palkia, you'll be transported out of the pink rift and back into Spear Pillar. You'll still see the pink rift, but it's empty.