After defeating Byron, head to the Canalave Library. You'll meet Professor Rowan, Dawn/Lucas, and Barry. They'll discuss the legends of Sinnoh. Suddenly, an earthquake strikes, and you'll learn that Team Galactic has attacked Lake Valor. Fly to Pastoria City and head east to Lake Valor. You'll find the lake drained and Magikarp flopping around. Commander Saturn will be there. Battle him!
After defeating Saturn, he'll reveal Team Galactic's plans. Now, head back to Canalave City. A sailor near the docks can take you to Iron Island. On Iron Island, you'll meet Riley, who will join you as a tag partner. Work your way through the cave, battling trainers and Team Galactic Grunts. Riley will heal your Pokémon after every battle. At the end, you'll receive a Riolu Egg. This is an excellent opportunity to get a powerful Fighting/Steel-type Pokémon!